text: /*
   *  fs/eventpoll.c (Efficient event retrieval implementation)
   *  Copyright (C) 2001,...,2009	 Davide Libenzi
   *
   *  This program is free software; you can redistribute it and/or modify
   *  it under the terms of the GNU General Public License as published by
   *  the Free Software Foundation; either version 2 of the License, or
   *  (at your option) any later version.
   *
   *  Davide Libenzi <davidel@xmailserver.org>
   *
   */
  
  #include <linux/init.h>
  #include <linux/kernel.h>
  #include <linux/sched.h>
  #include <linux/fs.h>
  #include <linux/file.h>
  #include <linux/signal.h>
  #include <linux/errno.h>
  #include <linux/mm.h>
  #include <linux/slab.h>
  #include <linux/poll.h>
  #include <linux/string.h>
  #include <linux/list.h>
  #include <linux/hash.h>
  #include <linux/spinlock.h>
  #include <linux/syscalls.h>
  #include <linux/rbtree.h>
  #include <linux/wait.h>
  #include <linux/eventpoll.h>
  #include <linux/mount.h>
  #include <linux/bitops.h>
  #include <linux/mutex.h>
  #include <linux/anon_inodes.h>
  #include <linux/device.h>
  #include <asm/uaccess.h>
  #include <asm/io.h>
  #include <asm/mman.h>
  #include <linux/atomic.h>
  #include <linux/proc_fs.h>
  #include <linux/seq_file.h>
  #include <linux/compat.h>
  #include <linux/rculist.h>
  
  /*
   * LOCKING:
   * There are three level of locking required by epoll :
   *
   * 1) epmutex (mutex)
   * 2) ep->mtx (mutex)
   * 3) ep->lock (spinlock)
   *
   * The acquire order is the one listed above, from 1 to 3.
   * We need a spinlock (ep->lock) because we manipulate objects
   * from inside the poll callback, that might be triggered from
   * a wake_up() that in turn might be called from IRQ context.
   * So we can't sleep inside the poll callback and hence we need
   * a spinlock. During the event transfer loop (from kernel to
   * user space) we could end up sleeping due a copy_to_user(), so
   * we need a lock that will allow us to sleep. This lock is a
   * mutex (ep->mtx). It is acquired during the event transfer loop,
   * during epoll_ctl(EPOLL_CTL_DEL) and during eventpoll_release_file().
   * Then we also need a global mutex to serialize eventpoll_release_file()
   * and ep_free().
   * This mutex is acquired by ep_free() during the epoll file
   * cleanup path and it is also acquired by eventpoll_release_file()
   * if a file has been pushed inside an epoll set and it is then
   * close()d without a previous call to epoll_ctl(EPOLL_CTL_DEL).
   * It is also acquired when inserting an epoll fd onto another epoll
   * fd. We do this so that we walk the epoll tree and ensure that this
   * insertion does not create a cycle of epoll file descriptors, which
   * could lead to deadlock. We need a global mutex to prevent two
   * simultaneous inserts (A into B and B into A) from racing and
   * constructing a cycle without either insert observing that it is
   * going to.
   * It is necessary to acquire multiple "ep->mtx"es at once in the
   * case when one epoll fd is added to another. In this case, we
   * always acquire the locks in the order of nesting (i.e. after
   * epoll_ctl(e1, EPOLL_CTL_ADD, e2), e1->mtx will always be acquired
   * before e2->mtx). Since we disallow cycles of epoll file
   * descriptors, this ensures that the mutexes are well-ordered. In
   * order to communicate this nesting to lockdep, when walking a tree
   * of epoll file descriptors, we use the current recursion depth as
   * the lockdep subkey.
   * It is possible to drop the "ep->mtx" and to use the global
   * mutex "epmutex" (together with "ep->lock") to have it working,
   * but having "ep->mtx" will make the interface more scalable.
   * Events that require holding "epmutex" are very rare, while for
   * normal operations the epoll private "ep->mtx" will guarantee
   * a better scalability.
   */
  
  /* Epoll private bits inside the event mask */
  #define EP_PRIVATE_BITS (EPOLLWAKEUP | EPOLLONESHOT | EPOLLET | EPOLLEXCLUSIVE)
  
  #define EPOLLINOUT_BITS (POLLIN | POLLOUT)
  
  #define EPOLLEXCLUSIVE_OK_BITS (EPOLLINOUT_BITS | POLLERR | POLLHUP | \
  				EPOLLWAKEUP | EPOLLET | EPOLLEXCLUSIVE)
  
  /* Maximum number of nesting allowed inside epoll sets */
  #define EP_MAX_NESTS 4
  
  #define EP_MAX_EVENTS (INT_MAX / sizeof(struct epoll_event))
  
  #define EP_UNACTIVE_PTR ((void *) -1L)
  
  #define EP_ITEM_COST (sizeof(struct epitem) + sizeof(struct eppoll_entry))
  
  struct epoll_filefd {
  	struct file *file;
  	int fd;
  } __packed;
  
  /*
   * Structure used to track possible nested calls, for too deep recursions
   * and loop cycles.
   */
  struct nested_call_node {
  	struct list_head llink;
  	void *cookie;
  	void *ctx;
  };
  
  /*
   * This structure is used as collector for nested calls, to check for
   * maximum recursion dept and loop cycles.
   */
  struct nested_calls {
  	struct list_head tasks_call_list;
  	spinlock_t lock;
  };
  
  /*
   * Each file descriptor added to the eventpoll interface will
   * have an entry of this type linked to the "rbr" RB tree.
   * Avoid increasing the size of this struct, there can be many thousands
   * of these on a server and we do not want this to take another cache line.
   */
  struct epitem {
  	union {
  		/* RB tree node links this structure to the eventpoll RB tree */
  		struct rb_node rbn;
  		/* Used to free the struct epitem */
  		struct rcu_head rcu;
  	};
  
  	/* List header used to link this structure to the eventpoll ready list */
  	struct list_head rdllink;
  
  	/*
  	 * Works together "struct eventpoll"->ovflist in keeping the
  	 * single linked chain of items.
  	 */
  	struct epitem *next;
  
  	/* The file descriptor information this item refers to */
  	struct epoll_filefd ffd;
  
  	/* Number of active wait queue attached to poll operations */
  	int nwait;
  
  	/* List containing poll wait queues */
  	struct list_head pwqlist;
  
  	/* The "container" of this item */
  	struct eventpoll *ep;
  
  	/* List header used to link this item to the "struct file" items list */
  	struct list_head fllink;
  
  	/* wakeup_source used when EPOLLWAKEUP is set */
  	struct wakeup_source __rcu *ws;
  
  	/* The structure that describe the interested events and the source fd */
  	struct epoll_event event;
  };
  
  /*
   * This structure is stored inside the "private_data" member of the file
   * structure and represents the main data structure for the eventpoll
   * interface.
   */
  struct eventpoll {
  	/* Protect the access to this structure */
  	spinlock_t lock;
  
  	/*
  	 * This mutex is used to ensure that files are not removed
  	 * while epoll is using them. This is held during the event
  	 * collection loop, the file cleanup path, the epoll file exit
  	 * code and the ctl operations.
  	 */
  	struct mutex mtx;
  
  	/* Wait queue used by sys_epoll_wait() */
  	wait_queue_head_t wq;
  
  	/* Wait queue used by file->poll() */
  	wait_queue_head_t poll_wait;
  
  	/* List of ready file descriptors */
  	struct list_head rdllist;
  
  	/* RB tree root used to store monitored fd structs */
  	struct rb_root rbr;
  
  	/*
  	 * This is a single linked list that chains all the "struct epitem" that
  	 * happened while transferring ready events to userspace w/out
  	 * holding ->lock.
  	 */
  	struct epitem *ovflist;
  
  	/* wakeup_source used when ep_scan_ready_list is running */
  	struct wakeup_source *ws;
  
  	/* The user that created the eventpoll descriptor */
  	struct user_struct *user;
  
  	struct file *file;
  
  	/* used to optimize loop detection check */
  	int visited;
  	struct list_head visited_list_link;
  };
  
  /* Wait structure used by the poll hooks */
  struct eppoll_entry {
  	/* List header used to link this structure to the "struct epitem" */
  	struct list_head llink;
  
  	/* The "base" pointer is set to the container "struct epitem" */
  	struct epitem *base;
  
  	/*
  	 * Wait queue item that will be linked to the target file wait
  	 * queue head.
  	 */
  	wait_queue_t wait;
  
  	/* The wait queue head that linked the "wait" wait queue item */
  	wait_queue_head_t *whead;
  };
  
  /* Wrapper struct used by poll queueing */
  struct ep_pqueue {
  	poll_table pt;
  	struct epitem *epi;
  };
  
  /* Used by the ep_send_events() function as callback private data */
  struct ep_send_events_data {
  	int maxevents;
  	struct epoll_event __user *events;
  };
  
  /*
   * Configuration options available inside /proc/sys/fs/epoll/
   */
  /* Maximum number of epoll watched descriptors, per user */
  static long max_user_watches __read_mostly;
  
  /*
   * This mutex is used to serialize ep_free() and eventpoll_release_file().
   */
  static DEFINE_MUTEX(epmutex);
  
  /* Used to check for epoll file descriptor inclusion loops */
  static struct nested_calls poll_loop_ncalls;
  
  /* Used for safe wake up implementation */
  static struct nested_calls poll_safewake_ncalls;
  
  /* Used to call file's f_op->poll() under the nested calls boundaries */
  static struct nested_calls poll_readywalk_ncalls;
  
  /* Slab cache used to allocate "struct epitem" */
  static struct kmem_cache *epi_cache __read_mostly;
  
  /* Slab cache used to allocate "struct eppoll_entry" */
  static struct kmem_cache *pwq_cache __read_mostly;
  
  /* Visited nodes during ep_loop_check(), so we can unset them when we finish */
  static LIST_HEAD(visited_list);
  
  /*
   * List of files with newly added links, where we may need to limit the number
   * of emanating paths. Protected by the epmutex.
   */
  static LIST_HEAD(tfile_check_list);
  
  #ifdef CONFIG_SYSCTL
  
  #include <linux/sysctl.h>
  
  static long zero;
  static long long_max = LONG_MAX;
  
  struct ctl_table epoll_table[] = {
  	{
  		.procname	= "max_user_watches",
  		.data		= &max_user_watches,
  		.maxlen		= sizeof(max_user_watches),
  		.mode		= 0644,
  		.proc_handler	= proc_doulongvec_minmax,
  		.extra1		= &zero,
  		.extra2		= &long_max,
  	},
  	{ }
  };
  #endif /* CONFIG_SYSCTL */
  
  static const struct file_operations eventpoll_fops;
  
  static inline int is_file_epoll(struct file *f)
  {
  	return f->f_op == &eventpoll_fops;
  }
  
  /* Setup the structure that is used as key for the RB tree */
  static inline void ep_set_ffd(struct epoll_filefd *ffd,
  			      struct file *file, int fd)
  {
  	ffd->file = file;
  	ffd->fd = fd;
  }
  
  /* Compare RB tree keys */
  static inline int ep_cmp_ffd(struct epoll_filefd *p1,
  			     struct epoll_filefd *p2)
  {
  	return (p1->file > p2->file ? +1:
  	        (p1->file < p2->file ? -1 : p1->fd - p2->fd));
  }
  
  /* Tells us if the item is currently linked */
  static inline int ep_is_linked(struct list_head *p)
  {
  	return !list_empty(p);
  }
  
  static inline struct eppoll_entry *ep_pwq_from_wait(wait_queue_t *p)
  {
  	return container_of(p, struct eppoll_entry, wait);
  }
  
  /* Get the "struct epitem" from a wait queue pointer */
  static inline struct epitem *ep_item_from_wait(wait_queue_t *p)
  {
  	return container_of(p, struct eppoll_entry, wait)->base;
  }
  
  /* Get the "struct epitem" from an epoll queue wrapper */
  static inline struct epitem *ep_item_from_epqueue(poll_table *p)
  {
  	return container_of(p, struct ep_pqueue, pt)->epi;
  }
  
  /* Tells if the epoll_ctl(2) operation needs an event copy from userspace */
  static inline int ep_op_has_event(int op)
  {
  	return op != EPOLL_CTL_DEL;
  }
  
  /* Initialize the poll safe wake up structure */
  static void ep_nested_calls_init(struct nested_calls *ncalls)
  {
  	INIT_LIST_HEAD(&ncalls->tasks_call_list);
  	spin_lock_init(&ncalls->lock);
  }
  
  /**
   * ep_events_available - Checks if ready events might be available.
   *
   * @ep: Pointer to the eventpoll context.
   *
   * Returns: Returns a value different than zero if ready events are available,
   *          or zero otherwise.
   */
  static inline int ep_events_available(struct eventpoll *ep)
  {
  	return !list_empty(&ep->rdllist) || ep->ovflist != EP_UNACTIVE_PTR;
  }
  
  /**
   * ep_call_nested - Perform a bound (possibly) nested call, by checking
   *                  that the recursion limit is not exceeded, and that
   *                  the same nested call (by the meaning of same cookie) is
   *                  no re-entered.
   *
   * @ncalls: Pointer to the nested_calls structure to be used for this call.
   * @max_nests: Maximum number of allowed nesting calls.
   * @nproc: Nested call core function pointer.
   * @priv: Opaque data to be passed to the @nproc callback.
   * @cookie: Cookie to be used to identify this nested call.
   * @ctx: This instance context.
   *
   * Returns: Returns the code returned by the @nproc callback, or -1 if
   *          the maximum recursion limit has been exceeded.
   */
  static int ep_call_nested(struct nested_calls *ncalls, int max_nests,
  			  int (*nproc)(void *, void *, int), void *priv,
  			  void *cookie, void *ctx)
  {
  	int error, call_nests = 0;
  	unsigned long flags;
  	struct list_head *lsthead = &ncalls->tasks_call_list;
  	struct nested_call_node *tncur;
  	struct nested_call_node tnode;
  
  	spin_lock_irqsave(&ncalls->lock, flags);
  
  	/*
  	 * Try to see if the current task is already inside this wakeup call.
  	 * We use a list here, since the population inside this set is always
  	 * very much limited.
  	 */
  	list_for_each_entry(tncur, lsthead, llink) {
  		if (tncur->ctx == ctx &&
  		    (tncur->cookie == cookie || ++call_nests > max_nests)) {
  			/*
  			 * Ops ... loop detected or maximum nest level reached.
  			 * We abort this wake by breaking the cycle itself.
  			 */
  			error = -1;
  			goto out_unlock;
  		}
  	}
  
  	/* Add the current task and cookie to the list */
  	tnode.ctx = ctx;
  	tnode.cookie = cookie;
  	list_add(&tnode.llink, lsthead);
  
  	spin_unlock_irqrestore(&ncalls->lock, flags);
  
  	/* Call the nested function */
  	error = (*nproc)(priv, cookie, call_nests);
  
  	/* Remove the current task from the list */
  	spin_lock_irqsave(&ncalls->lock, flags);
  	list_del(&tnode.llink);
  out_unlock:
  	spin_unlock_irqrestore(&ncalls->lock, flags);
  
  	return error;
  }
  
  /*
   * As described in commit 0ccf831cb lockdep: annotate epoll
   * the use of wait queues used by epoll is done in a very controlled
   * manner. Wake ups can nest inside each other, but are never done
   * with the same locking. For example:
   *
   *   dfd = socket(...);
   *   efd1 = epoll_create();
   *   efd2 = epoll_create();
   *   epoll_ctl(efd1, EPOLL_CTL_ADD, dfd, ...);
   *   epoll_ctl(efd2, EPOLL_CTL_ADD, efd1, ...);
   *
   * When a packet arrives to the device underneath "dfd", the net code will
   * issue a wake_up() on its poll wake list. Epoll (efd1) has installed a
   * callback wakeup entry on that queue, and the wake_up() performed by the
   * "dfd" net code will end up in ep_poll_callback(). At this point epoll
   * (efd1) notices that it may have some event ready, so it needs to wake up
   * the waiters on its poll wait list (efd2). So it calls ep_poll_safewake()
   * that ends up in another wake_up(), after having checked about the
   * recursion constraints. That are, no more than EP_MAX_POLLWAKE_NESTS, to
   * avoid stack blasting.
   *
   * When CONFIG_DEBUG_LOCK_ALLOC is enabled, make sure lockdep can handle
   * this special case of epoll.
   */
  #ifdef CONFIG_DEBUG_LOCK_ALLOC
  static inline void ep_wake_up_nested(wait_queue_head_t *wqueue,
  				     unsigned long events, int subclass)
  {
  	unsigned long flags;
  
  	spin_lock_irqsave_nested(&wqueue->lock, flags, subclass);
  	wake_up_locked_poll(wqueue, events);
  	spin_unlock_irqrestore(&wqueue->lock, flags);
  }
  #else
  static inline void ep_wake_up_nested(wait_queue_head_t *wqueue,
  				     unsigned long events, int subclass)
  {
  	wake_up_poll(wqueue, events);
  }
  #endif
  
  static int ep_poll_wakeup_proc(void *priv, void *cookie, int call_nests)
  {
  	ep_wake_up_nested((wait_queue_head_t *) cookie, POLLIN,
  			  1 + call_nests);
  	return 0;
  }
  
  /*
   * Perform a safe wake up of the poll wait list. The problem is that
   * with the new callback'd wake up system, it is possible that the
   * poll callback is reentered from inside the call to wake_up() done
   * on the poll wait queue head. The rule is that we cannot reenter the
   * wake up code from the same task more than EP_MAX_NESTS times,
   * and we cannot reenter the same wait queue head at all. This will
   * enable to have a hierarchy of epoll file descriptor of no more than
   * EP_MAX_NESTS deep.
   */
  static void ep_poll_safewake(wait_queue_head_t *wq)
  {
  	int this_cpu = get_cpu();
  
  	ep_call_nested(&poll_safewake_ncalls, EP_MAX_NESTS,
  		       ep_poll_wakeup_proc, NULL, wq, (void *) (long) this_cpu);
  
  	put_cpu();
  }
  
  static void ep_remove_wait_queue(struct eppoll_entry *pwq)
  {
  	wait_queue_head_t *whead;
  
  	rcu_read_lock();
  	/* If it is cleared by POLLFREE, it should be rcu-safe */
  	whead = rcu_dereference(pwq->whead);
  	if (whead)
  		remove_wait_queue(whead, &pwq->wait);
  	rcu_read_unlock();
  }
  
  /*
   * This function unregisters poll callbacks from the associated file
   * descriptor.  Must be called with "mtx" held (or "epmutex" if called from
   * ep_free).
   */
  static void ep_unregister_pollwait(struct eventpoll *ep, struct epitem *epi)
  {
  	struct list_head *lsthead = &epi->pwqlist;
  	struct eppoll_entry *pwq;
  
  	while (!list_empty(lsthead)) {
  		pwq = list_first_entry(lsthead, struct eppoll_entry, llink);
  
  		list_del(&pwq->llink);
  		ep_remove_wait_queue(pwq);
  		kmem_cache_free(pwq_cache, pwq);
  	}
  }
  
  /* call only when ep->mtx is held */
  static inline struct wakeup_source *ep_wakeup_source(struct epitem *epi)
  {
  	return rcu_dereference_check(epi->ws, lockdep_is_held(&epi->ep->mtx));
  }
  
  /* call only when ep->mtx is held */
  static inline void ep_pm_stay_awake(struct epitem *epi)
  {
  	struct wakeup_source *ws = ep_wakeup_source(epi);
  
  	if (ws)
  		__pm_stay_awake(ws);
  }
  
  static inline bool ep_has_wakeup_source(struct epitem *epi)
  {
  	return rcu_access_pointer(epi->ws) ? true : false;
  }
  
  /* call when ep->mtx cannot be held (ep_poll_callback) */
  static inline void ep_pm_stay_awake_rcu(struct epitem *epi)
  {
  	struct wakeup_source *ws;
  
  	rcu_read_lock();
  	ws = rcu_dereference(epi->ws);
  	if (ws)
  		__pm_stay_awake(ws);
  	rcu_read_unlock();
  }
  
  /**
   * ep_scan_ready_list - Scans the ready list in a way that makes possible for
   *                      the scan code, to call f_op->poll(). Also allows for
   *                      O(NumReady) performance.
   *
   * @ep: Pointer to the epoll private data structure.
   * @sproc: Pointer to the scan callback.
   * @priv: Private opaque data passed to the @sproc callback.
   * @depth: The current depth of recursive f_op->poll calls.
   * @ep_locked: caller already holds ep->mtx
   *
   * Returns: The same integer error code returned by the @sproc callback.
   */
  static int ep_scan_ready_list(struct eventpoll *ep,
  			      int (*sproc)(struct eventpoll *,
  					   struct list_head *, void *),
  			      void *priv, int depth, bool ep_locked)
  {
  	int error, pwake = 0;
  	unsigned long flags;
  	struct epitem *epi, *nepi;
  	LIST_HEAD(txlist);
  
  	/*
  	 * We need to lock this because we could be hit by
  	 * eventpoll_release_file() and epoll_ctl().
  	 */
  
  	if (!ep_locked)
  		mutex_lock_nested(&ep->mtx, depth);
  
  	/*
  	 * Steal the ready list, and re-init the original one to the
  	 * empty list. Also, set ep->ovflist to NULL so that events
  	 * happening while looping w/out locks, are not lost. We cannot
  	 * have the poll callback to queue directly on ep->rdllist,
  	 * because we want the "sproc" callback to be able to do it
  	 * in a lockless way.
  	 */
  	spin_lock_irqsave(&ep->lock, flags);
  	list_splice_init(&ep->rdllist, &txlist);
  	ep->ovflist = NULL;
  	spin_unlock_irqrestore(&ep->lock, flags);
  
  	/*
  	 * Now call the callback function.
  	 */
  	error = (*sproc)(ep, &txlist, priv);
  
  	spin_lock_irqsave(&ep->lock, flags);
  	/*
  	 * During the time we spent inside the "sproc" callback, some
  	 * other events might have been queued by the poll callback.
  	 * We re-insert them inside the main ready-list here.
  	 */
  	for (nepi = ep->ovflist; (epi = nepi) != NULL;
  	     nepi = epi->next, epi->next = EP_UNACTIVE_PTR) {
  		/*
  		 * We need to check if the item is already in the list.
  		 * During the "sproc" callback execution time, items are
  		 * queued into ->ovflist but the "txlist" might already
  		 * contain them, and the list_splice() below takes care of them.
  		 */
  		if (!ep_is_linked(&epi->rdllink)) {
  			list_add_tail(&epi->rdllink, &ep->rdllist);
  			ep_pm_stay_awake(epi);
  		}
  	}
  	/*
  	 * We need to set back ep->ovflist to EP_UNACTIVE_PTR, so that after
  	 * releasing the lock, events will be queued in the normal way inside
  	 * ep->rdllist.
  	 */
  	ep->ovflist = EP_UNACTIVE_PTR;
  
  	/*
  	 * Quickly re-inject items left on "txlist".
  	 */
  	list_splice(&txlist, &ep->rdllist);
  	__pm_relax(ep->ws);
  
  	if (!list_empty(&ep->rdllist)) {
  		/*
  		 * Wake up (if active) both the eventpoll wait list and
  		 * the ->poll() wait list (delayed after we release the lock).
  		 */
  		if (waitqueue_active(&ep->wq))
  			wake_up_locked(&ep->wq);
  		if (waitqueue_active(&ep->poll_wait))
  			pwake++;
  	}
  	spin_unlock_irqrestore(&ep->lock, flags);
  
  	if (!ep_locked)
  		mutex_unlock(&ep->mtx);
  
  	/* We have to call this outside the lock */
  	if (pwake)
  		ep_poll_safewake(&ep->poll_wait);
  
  	return error;
  }
  
  static void epi_rcu_free(struct rcu_head *head)
  {
  	struct epitem *epi = container_of(head, struct epitem, rcu);
  	kmem_cache_free(epi_cache, epi);
  }
  
  /*
   * Removes a "struct epitem" from the eventpoll RB tree and deallocates
   * all the associated resources. Must be called with "mtx" held.
   */
  static int ep_remove(struct eventpoll *ep, struct epitem *epi)
  {
  	unsigned long flags;
  	struct file *file = epi->ffd.file;
  
  	/*
  	 * Removes poll wait queue hooks. We _have_ to do this without holding
  	 * the "ep->lock" otherwise a deadlock might occur. This because of the
  	 * sequence of the lock acquisition. Here we do "ep->lock" then the wait
  	 * queue head lock when unregistering the wait queue. The wakeup callback
  	 * will run by holding the wait queue head lock and will call our callback
  	 * that will try to get "ep->lock".
  	 */
  	ep_unregister_pollwait(ep, epi);
  
  	/* Remove the current item from the list of epoll hooks */
  	spin_lock(&file->f_lock);
  	list_del_rcu(&epi->fllink);
  	spin_unlock(&file->f_lock);
  
  	rb_erase(&epi->rbn, &ep->rbr);
  
  	spin_lock_irqsave(&ep->lock, flags);
  	if (ep_is_linked(&epi->rdllink))
  		list_del_init(&epi->rdllink);
  	spin_unlock_irqrestore(&ep->lock, flags);
  
  	wakeup_source_unregister(ep_wakeup_source(epi));
  	/*
  	 * At this point it is safe to free the eventpoll item. Use the union
  	 * field epi->rcu, since we are trying to minimize the size of
  	 * 'struct epitem'. The 'rbn' field is no longer in use. Protected by
  	 * ep->mtx. The rcu read side, reverse_path_check_proc(), does not make
  	 * use of the rbn field.
  	 */
  	call_rcu(&epi->rcu, epi_rcu_free);
  
  	atomic_long_dec(&ep->user->epoll_watches);
  
  	return 0;
  }
  
  static void ep_free(struct eventpoll *ep)
  {
  	struct rb_node *rbp;
  	struct epitem *epi;
  
  	/* We need to release all tasks waiting for these file */
  	if (waitqueue_active(&ep->poll_wait))
  		ep_poll_safewake(&ep->poll_wait);
  
  	/*
  	 * We need to lock this because we could be hit by
  	 * eventpoll_release_file() while we're freeing the "struct eventpoll".
  	 * We do not need to hold "ep->mtx" here because the epoll file
  	 * is on the way to be removed and no one has references to it
  	 * anymore. The only hit might come from eventpoll_release_file() but
  	 * holding "epmutex" is sufficient here.
  	 */
  	mutex_lock(&epmutex);
  
  	/*
  	 * Walks through the whole tree by unregistering poll callbacks.
  	 */
  	for (rbp = rb_first(&ep->rbr); rbp; rbp = rb_next(rbp)) {
  		epi = rb_entry(rbp, struct epitem, rbn);
  
  		ep_unregister_pollwait(ep, epi);
  		cond_resched();
  	}
  
  	/*
  	 * Walks through the whole tree by freeing each "struct epitem". At this
  	 * point we are sure no poll callbacks will be lingering around, and also by
  	 * holding "epmutex" we can be sure that no file cleanup code will hit
  	 * us during this operation. So we can avoid the lock on "ep->lock".
  	 * We do not need to lock ep->mtx, either, we only do it to prevent
  	 * a lockdep warning.
  	 */
  	mutex_lock(&ep->mtx);
  	while ((rbp = rb_first(&ep->rbr)) != NULL) {
  		epi = rb_entry(rbp, struct epitem, rbn);
  		ep_remove(ep, epi);
  		cond_resched();
  	}
  	mutex_unlock(&ep->mtx);
  
  	mutex_unlock(&epmutex);
  	mutex_destroy(&ep->mtx);
  	free_uid(ep->user);
  	wakeup_source_unregister(ep->ws);
  	kfree(ep);
  }
  
  static int ep_eventpoll_release(struct inode *inode, struct file *file)
  {
  	struct eventpoll *ep = file->private_data;
  
  	if (ep)
  		ep_free(ep);
  
  	return 0;
  }
  
  static inline unsigned int ep_item_poll(struct epitem *epi, poll_table *pt)
  {
  	pt->_key = epi->event.events;
  
  	return epi->ffd.file->f_op->poll(epi->ffd.file, pt) & epi->event.events;
  }
  
  static int ep_read_events_proc(struct eventpoll *ep, struct list_head *head,
  			       void *priv)
  {
  	struct epitem *epi, *tmp;
  	poll_table pt;
  
  	init_poll_funcptr(&pt, NULL);
  
  	list_for_each_entry_safe(epi, tmp, head, rdllink) {
  		if (ep_item_poll(epi, &pt))
  			return POLLIN | POLLRDNORM;
  		else {
  			/*
  			 * Item has been dropped into the ready list by the poll
  			 * callback, but it's not actually ready, as far as
  			 * caller requested events goes. We can remove it here.
  			 */
  			__pm_relax(ep_wakeup_source(epi));
  			list_del_init(&epi->rdllink);
  		}
  	}
  
  	return 0;
  }
  
  static void ep_ptable_queue_proc(struct file *file, wait_queue_head_t *whead,
  				 poll_table *pt);
  
  struct readyevents_arg {
  	struct eventpoll *ep;
  	bool locked;
  };
  
  static int ep_poll_readyevents_proc(void *priv, void *cookie, int call_nests)
  {
  	struct readyevents_arg *arg = priv;
  
  	return ep_scan_ready_list(arg->ep, ep_read_events_proc, NULL,
  				  call_nests + 1, arg->locked);
  }
  
  static unsigned int ep_eventpoll_poll(struct file *file, poll_table *wait)
  {
  	int pollflags;
  	struct eventpoll *ep = file->private_data;
  	struct readyevents_arg arg;
  
  	/*
  	 * During ep_insert() we already hold the ep->mtx for the tfile.
  	 * Prevent re-aquisition.
  	 */
  	arg.locked = wait && (wait->_qproc == ep_ptable_queue_proc);
  	arg.ep = ep;
  
  	/* Insert inside our poll wait queue */
  	poll_wait(file, &ep->poll_wait, wait);
  
  	/*
  	 * Proceed to find out if wanted events are really available inside
  	 * the ready list. This need to be done under ep_call_nested()
  	 * supervision, since the call to f_op->poll() done on listed files
  	 * could re-enter here.
  	 */
  	pollflags = ep_call_nested(&poll_readywalk_ncalls, EP_MAX_NESTS,
  				   ep_poll_readyevents_proc, &arg, ep, current);
  
  	return pollflags != -1 ? pollflags : 0;
  }
  
  #ifdef CONFIG_PROC_FS
  static void ep_show_fdinfo(struct seq_file *m, struct file *f)
  {
  	struct eventpoll *ep = f->private_data;
  	struct rb_node *rbp;
  
  	mutex_lock(&ep->mtx);
  	for (rbp = rb_first(&ep->rbr); rbp; rbp = rb_next(rbp)) {
  		struct epitem *epi = rb_entry(rbp, struct epitem, rbn);
  
  		seq_printf(m, "tfd: %8d events: %8x data: %16llx\n",
  			   epi->ffd.fd, epi->event.events,
  			   (long long)epi->event.data);
  		if (seq_has_overflowed(m))
  			break;
  	}
  	mutex_unlock(&ep->mtx);
  }
  #endif
  
  /* File callbacks that implement the eventpoll file behaviour */
  static const struct file_operations eventpoll_fops = {
  #ifdef CONFIG_PROC_FS
  	.show_fdinfo	= ep_show_fdinfo,
  #endif
  	.release	= ep_eventpoll_release,
  	.poll		= ep_eventpoll_poll,
  	.llseek		= noop_llseek,
  };
  
  /*
   * This is called from eventpoll_release() to unlink files from the eventpoll
   * interface. We need to have this facility to cleanup correctly files that are
   * closed without being removed from the eventpoll interface.
   */
  void eventpoll_release_file(struct file *file)
  {
  	struct eventpoll *ep;
  	struct epitem *epi, *next;
  
  	/*
  	 * We don't want to get "file->f_lock" because it is not
  	 * necessary. It is not necessary because we're in the "struct file"
  	 * cleanup path, and this means that no one is using this file anymore.
  	 * So, for example, epoll_ctl() cannot hit here since if we reach this
  	 * point, the file counter already went to zero and fget() would fail.
  	 * The only hit might come from ep_free() but by holding the mutex
  	 * will correctly serialize the operation. We do need to acquire
  	 * "ep->mtx" after "epmutex" because ep_remove() requires it when called
  	 * from anywhere but ep_free().
  	 *
  	 * Besides, ep_remove() acquires the lock, so we can't hold it here.
  	 */
  	mutex_lock(&epmutex);
  	list_for_each_entry_safe(epi, next, &file->f_ep_links, fllink) {
  		ep = epi->ep;
  		mutex_lock_nested(&ep->mtx, 0);
  		ep_remove(ep, epi);
  		mutex_unlock(&ep->mtx);
  	}
  	mutex_unlock(&epmutex);
  }
  
  static int ep_alloc(struct eventpoll **pep)
  {
  	int error;
  	struct user_struct *user;
  	struct eventpoll *ep;
  
  	user = get_current_user();
  	error = -ENOMEM;
  	ep = kzalloc(sizeof(*ep), GFP_KERNEL);
  	if (unlikely(!ep))
  		goto free_uid;
  
  	spin_lock_init(&ep->lock);
  	mutex_init(&ep->mtx);
  	init_waitqueue_head(&ep->wq);
  	init_waitqueue_head(&ep->poll_wait);
  	INIT_LIST_HEAD(&ep->rdllist);
  	ep->rbr = RB_ROOT;
  	ep->ovflist = EP_UNACTIVE_PTR;
  	ep->user = user;
  
  	*pep = ep;
  
  	return 0;
  
  free_uid:
  	free_uid(user);
  	return error;
  }
  
  /*
   * Search the file inside the eventpoll tree. The RB tree operations
   * are protected by the "mtx" mutex, and ep_find() must be called with
   * "mtx" held.
   */
  static struct epitem *ep_find(struct eventpoll *ep, struct file *file, int fd)
  {
  	int kcmp;
  	struct rb_node *rbp;
  	struct epitem *epi, *epir = NULL;
  	struct epoll_filefd ffd;
  
  	ep_set_ffd(&ffd, file, fd);
  	for (rbp = ep->rbr.rb_node; rbp; ) {
  		epi = rb_entry(rbp, struct epitem, rbn);
  		kcmp = ep_cmp_ffd(&ffd, &epi->ffd);
  		if (kcmp > 0)
  			rbp = rbp->rb_right;
  		else if (kcmp < 0)
  			rbp = rbp->rb_left;
  		else {
  			epir = epi;
  			break;
  		}
  	}
  
  	return epir;
  }
  
  /*
   * This is the callback that is passed to the wait queue wakeup
   * mechanism. It is called by the stored file descriptors when they
   * have events to report.
   */
  static int ep_poll_callback(wait_queue_t *wait, unsigned mode, int sync, void *key)
  {
  	int pwake = 0;
  	unsigned long flags;
  	struct epitem *epi = ep_item_from_wait(wait);
  	struct eventpoll *ep = epi->ep;
  	int ewake = 0;
  
  	if ((unsigned long)key & POLLFREE) {
  		ep_pwq_from_wait(wait)->whead = NULL;
  		/*
  		 * whead = NULL above can race with ep_remove_wait_queue()
  		 * which can do another remove_wait_queue() after us, so we
  		 * can't use __remove_wait_queue(). whead->lock is held by
  		 * the caller.
  		 */
  		list_del_init(&wait->task_list);
  	}
  
  	spin_lock_irqsave(&ep->lock, flags);
  
  	/*
  	 * If the event mask does not contain any poll(2) event, we consider the
  	 * descriptor to be disabled. This condition is likely the effect of the
  	 * EPOLLONESHOT bit that disables the descriptor when an event is received,
  	 * until the next EPOLL_CTL_MOD will be issued.
  	 */
  	if (!(epi->event.events & ~EP_PRIVATE_BITS))
  		goto out_unlock;
  
  	/*
  	 * Check the events coming with the callback. At this stage, not
  	 * every device reports the events in the "key" parameter of the
  	 * callback. We need to be able to handle both cases here, hence the
  	 * test for "key" != NULL before the event match test.
  	 */
  	if (key && !((unsigned long) key & epi->event.events))
  		goto out_unlock;
  
  	/*
  	 * If we are transferring events to userspace, we can hold no locks
  	 * (because we're accessing user memory, and because of linux f_op->poll()
  	 * semantics). All the events that happen during that period of time are
  	 * chained in ep->ovflist and requeued later on.
  	 */
  	if (unlikely(ep->ovflist != EP_UNACTIVE_PTR)) {
  		if (epi->next == EP_UNACTIVE_PTR) {
  			epi->next = ep->ovflist;
  			ep->ovflist = epi;
  			if (epi->ws) {
  				/*
  				 * Activate ep->ws since epi->ws may get
  				 * deactivated at any time.
  				 */
  				__pm_stay_awake(ep->ws);
  			}
  
  		}
  		goto out_unlock;
  	}
  
  	/* If this file is already in the ready list we exit soon */
  	if (!ep_is_linked(&epi->rdllink)) {
  		list_add_tail(&epi->rdllink, &ep->rdllist);
  		ep_pm_stay_awake_rcu(epi);
  	}
  
  	/*
  	 * Wake up ( if active ) both the eventpoll wait list and the ->poll()
  	 * wait list.
  	 */
  	if (waitqueue_active(&ep->wq)) {
  		if ((epi->event.events & EPOLLEXCLUSIVE) &&
  					!((unsigned long)key & POLLFREE)) {
  			switch ((unsigned long)key & EPOLLINOUT_BITS) {
  			case POLLIN:
  				if (epi->event.events & POLLIN)
  					ewake = 1;
  				break;
  			case POLLOUT:
  				if (epi->event.events & POLLOUT)
  					ewake = 1;
  				break;
  			case 0:
  				ewake = 1;
  				break;
  			}
  		}
  		wake_up_locked(&ep->wq);
  	}
  	if (waitqueue_active(&ep->poll_wait))
  		pwake++;
  
  out_unlock:
  	spin_unlock_irqrestore(&ep->lock, flags);
  
  	/* We have to call this outside the lock */
  	if (pwake)
  		ep_poll_safewake(&ep->poll_wait);
  
  	if (epi->event.events & EPOLLEXCLUSIVE)
  		return ewake;
  
  	return 1;
  }
  
  /*
   * This is the callback that is used to add our wait queue to the
   * target file wakeup lists.
   */
  static void ep_ptable_queue_proc(struct file *file, wait_queue_head_t *whead,
  				 poll_table *pt)
  {
  	struct epitem *epi = ep_item_from_epqueue(pt);
  	struct eppoll_entry *pwq;
  
  	if (epi->nwait >= 0 && (pwq = kmem_cache_alloc(pwq_cache, GFP_KERNEL))) {
  		init_waitqueue_func_entry(&pwq->wait, ep_poll_callback);
  		pwq->whead = whead;
  		pwq->base = epi;
  		if (epi->event.events & EPOLLEXCLUSIVE)
  			add_wait_queue_exclusive(whead, &pwq->wait);
  		else
  			add_wait_queue(whead, &pwq->wait);
  		list_add_tail(&pwq->llink, &epi->pwqlist);
  		epi->nwait++;
  	} else {
  		/* We have to signal that an error occurred */
  		epi->nwait = -1;
  	}
  }
  
  static void ep_rbtree_insert(struct eventpoll *ep, struct epitem *epi)
  {
  	int kcmp;
  	struct rb_node **p = &ep->rbr.rb_node, *parent = NULL;
  	struct epitem *epic;
  
  	while (*p) {
  		parent = *p;
  		epic = rb_entry(parent, struct epitem, rbn);
  		kcmp = ep_cmp_ffd(&epi->ffd, &epic->ffd);
  		if (kcmp > 0)
  			p = &parent->rb_right;
  		else
  			p = &parent->rb_left;
  	}
  	rb_link_node(&epi->rbn, parent, p);
  	rb_insert_color(&epi->rbn, &ep->rbr);
  }
  
  
  
  #define PATH_ARR_SIZE 5
  /*
   * These are the number paths of length 1 to 5, that we are allowing to emanate
   * from a single file of interest. For example, we allow 1000 paths of length
   * 1, to emanate from each file of interest. This essentially represents the
   * potential wakeup paths, which need to be limited in order to avoid massive
   * uncontrolled wakeup storms. The common use case should be a single ep which
   * is connected to n file sources. In this case each file source has 1 path
   * of length 1. Thus, the numbers below should be more than sufficient. These
   * path limits are enforced during an EPOLL_CTL_ADD operation, since a modify
   * and delete can't add additional paths. Protected by the epmutex.
   */
  static const int path_limits[PATH_ARR_SIZE] = { 1000, 500, 100, 50, 10 };
  static int path_count[PATH_ARR_SIZE];
  
  static int path_count_inc(int nests)
  {
  	/* Allow an arbitrary number of depth 1 paths */
  	if (nests == 0)
  		return 0;
  
  	if (++path_count[nests] > path_limits[nests])
  		return -1;
  	return 0;
  }
  
  static void path_count_init(void)
  {
  	int i;
  
  	for (i = 0; i < PATH_ARR_SIZE; i++)
  		path_count[i] = 0;
  }
  
  static int reverse_path_check_proc(void *priv, void *cookie, int call_nests)
  {
  	int error = 0;
  	struct file *file = priv;
  	struct file *child_file;
  	struct epitem *epi;
  
  	/* CTL_DEL can remove links here, but that can't increase our count */
  	rcu_read_lock();
  	list_for_each_entry_rcu(epi, &file->f_ep_links, fllink) {
  		child_file = epi->ep->file;
  		if (is_file_epoll(child_file)) {
  			if (list_empty(&child_file->f_ep_links)) {
  				if (path_count_inc(call_nests)) {
  					error = -1;
  					break;
  				}
  			} else {
  				error = ep_call_nested(&poll_loop_ncalls,
  							EP_MAX_NESTS,
  							reverse_path_check_proc,
  							child_file, child_file,
  							current);
  			}
  			if (error != 0)
  				break;
  		} else {
  			printk(KERN_ERR "reverse_path_check_proc: "
  				"file is not an ep!\n");
  		}
  	}
  	rcu_read_unlock();
  	return error;
  }
  
  /**
   * reverse_path_check - The tfile_check_list is list of file *, which have
   *                      links that are proposed to be newly added. We need to
   *                      make sure that those added links don't add too many
   *                      paths such that we will spend all our time waking up
   *                      eventpoll objects.
   *
   * Returns: Returns zero if the proposed links don't create too many paths,
   *	    -1 otherwise.
   */
  static int reverse_path_check(void)
  {
  	int error = 0;
  	struct file *current_file;
  
  	/* let's call this for all tfiles */
  	list_for_each_entry(current_file, &tfile_check_list, f_tfile_llink) {
  		path_count_init();
  		error = ep_call_nested(&poll_loop_ncalls, EP_MAX_NESTS,
  					reverse_path_check_proc, current_file,
  					current_file, current);
  		if (error)
  			break;
  	}
  	return error;
  }
  
  static int ep_create_wakeup_source(struct epitem *epi)
  {
  	const char *name;
  	struct wakeup_source *ws;
  
  	if (!epi->ep->ws) {
  		epi->ep->ws = wakeup_source_register("eventpoll");
  		if (!epi->ep->ws)
  			return -ENOMEM;
  	}
  
  	name = epi->ffd.file->f_path.dentry->d_name.name;
  	ws = wakeup_source_register(name);
  
  	if (!ws)
  		return -ENOMEM;
  	rcu_assign_pointer(epi->ws, ws);
  
  	return 0;
  }
  
  /* rare code path, only used when EPOLL_CTL_MOD removes a wakeup source */
  static noinline void ep_destroy_wakeup_source(struct epitem *epi)
  {
  	struct wakeup_source *ws = ep_wakeup_source(epi);
  
  	RCU_INIT_POINTER(epi->ws, NULL);
  
  	/*
  	 * wait for ep_pm_stay_awake_rcu to finish, synchronize_rcu is
  	 * used internally by wakeup_source_remove, too (called by
  	 * wakeup_source_unregister), so we cannot use call_rcu
  	 */
  	synchronize_rcu();
  	wakeup_source_unregister(ws);
  }
  
  /*
   * Must be called with "mtx" held.
   */
  static int ep_insert(struct eventpoll *ep, struct epoll_event *event,
  		     struct file *tfile, int fd, int full_check)
  {
  	int error, revents, pwake = 0;
  	unsigned long flags;
  	long user_watches;
  	struct epitem *epi;
  	struct ep_pqueue epq;
  
  	user_watches = atomic_long_read(&ep->user->epoll_watches);
  	if (unlikely(user_watches >= max_user_watches))
  		return -ENOSPC;
  	if (!(epi = kmem_cache_alloc(epi_cache, GFP_KERNEL)))
  		return -ENOMEM;
  
  	/* Item initialization follow here ... */
  	INIT_LIST_HEAD(&epi->rdllink);
  	INIT_LIST_HEAD(&epi->fllink);
  	INIT_LIST_HEAD(&epi->pwqlist);
  	epi->ep = ep;
  	ep_set_ffd(&epi->ffd, tfile, fd);
  	epi->event = *event;
  	epi->nwait = 0;
  	epi->next = EP_UNACTIVE_PTR;
  	if (epi->event.events & EPOLLWAKEUP) {
  		error = ep_create_wakeup_source(epi);
  		if (error)
  			goto error_create_wakeup_source;
  	} else {
  		RCU_INIT_POINTER(epi->ws, NULL);
  	}
  
  	/* Initialize the poll table using the queue callback */
  	epq.epi = epi;
  	init_poll_funcptr(&epq.pt, ep_ptable_queue_proc);
  
  	/*
  	 * Attach the item to the poll hooks and get current event bits.
  	 * We can safely use the file* here because its usage count has
  	 * been increased by the caller of this function. Note that after
  	 * this operation completes, the poll callback can start hitting
  	 * the new item.
  	 */
  	revents = ep_item_poll(epi, &epq.pt);
  
  	/*
  	 * We have to check if something went wrong during the poll wait queue
  	 * install process. Namely an allocation for a wait queue failed due
  	 * high memory pressure.
  	 */
  	error = -ENOMEM;
  	if (epi->nwait < 0)
  		goto error_unregister;
  
  	/* Add the current item to the list of active epoll hook for this file */
  	spin_lock(&tfile->f_lock);
  	list_add_tail_rcu(&epi->fllink, &tfile->f_ep_links);
  	spin_unlock(&tfile->f_lock);
  
  	/*
  	 * Add the current item to the RB tree. All RB tree operations are
  	 * protected by "mtx", and ep_insert() is called with "mtx" held.
  	 */
  	ep_rbtree_insert(ep, epi);
  
  	/* now check if we've created too many backpaths */
  	error = -EINVAL;
  	if (full_check && reverse_path_check())
  		goto error_remove_epi;
  
  	/* We have to drop the new item inside our item list to keep track of it */
  	spin_lock_irqsave(&ep->lock, flags);
  
  	/* If the file is already "ready" we drop it inside the ready list */
  	if ((revents & event->events) && !ep_is_linked(&epi->rdllink)) {
  		list_add_tail(&epi->rdllink, &ep->rdllist);
  		ep_pm_stay_awake(epi);
  
  		/* Notify waiting tasks that events are available */
  		if (waitqueue_active(&ep->wq))
  			wake_up_locked(&ep->wq);
  		if (waitqueue_active(&ep->poll_wait))
  			pwake++;
  	}
  
  	spin_unlock_irqrestore(&ep->lock, flags);
  
  	atomic_long_inc(&ep->user->epoll_watches);
  
  	/* We have to call this outside the lock */
  	if (pwake)
  		ep_poll_safewake(&ep->poll_wait);
  
  	return 0;
  
  error_remove_epi:
  	spin_lock(&tfile->f_lock);
  	list_del_rcu(&epi->fllink);
  	spin_unlock(&tfile->f_lock);
  
  	rb_erase(&epi->rbn, &ep->rbr);
  
  error_unregister:
  	ep_unregister_pollwait(ep, epi);
  
  	/*
  	 * We need to do this because an event could have been arrived on some
  	 * allocated wait queue. Note that we don't care about the ep->ovflist
  	 * list, since that is used/cleaned only inside a section bound by "mtx".
  	 * And ep_insert() is called with "mtx" held.
  	 */
  	spin_lock_irqsave(&ep->lock, flags);
  	if (ep_is_linked(&epi->rdllink))
  		list_del_init(&epi->rdllink);
  	spin_unlock_irqrestore(&ep->lock, flags);
  
  	wakeup_source_unregister(ep_wakeup_source(epi));
  
  error_create_wakeup_source:
  	kmem_cache_free(epi_cache, epi);
  
  	return error;
  }
  
  /*
   * Modify the interest event mask by dropping an event if the new mask
   * has a match in the current file status. Must be called with "mtx" held.
   */
  static int ep_modify(struct eventpoll *ep, struct epitem *epi, struct epoll_event *event)
  {
  	int pwake = 0;
  	unsigned int revents;
  	poll_table pt;
  
  	init_poll_funcptr(&pt, NULL);
  
  	/*
  	 * Set the new event interest mask before calling f_op->poll();
  	 * otherwise we might miss an event that happens between the
  	 * f_op->poll() call and the new event set registering.
  	 */
  	epi->event.events = event->events; /* need barrier below */
  	epi->event.data = event->data; /* protected by mtx */
  	if (epi->event.events & EPOLLWAKEUP) {
  		if (!ep_has_wakeup_source(epi))
  			ep_create_wakeup_source(epi);
  	} else if (ep_has_wakeup_source(epi)) {
  		ep_destroy_wakeup_source(epi);
  	}
  
  	/*
  	 * The following barrier has two effects:
  	 *
  	 * 1) Flush epi changes above to other CPUs.  This ensures
  	 *    we do not miss events from ep_poll_callback if an
  	 *    event occurs immediately after we call f_op->poll().
  	 *    We need this because we did not take ep->lock while
  	 *    changing epi above (but ep_poll_callback does take
  	 *    ep->lock).
  	 *
  	 * 2) We also need to ensure we do not miss _past_ events
  	 *    when calling f_op->poll().  This barrier also
  	 *    pairs with the barrier in wq_has_sleeper (see
  	 *    comments for wq_has_sleeper).
  	 *
  	 * This barrier will now guarantee ep_poll_callback or f_op->poll
  	 * (or both) will notice the readiness of an item.
  	 */
  	smp_mb();
  
  	/*
  	 * Get current event bits. We can safely use the file* here because
  	 * its usage count has been increased by the caller of this function.
  	 */
  	revents = ep_item_poll(epi, &pt);
  
  	/*
  	 * If the item is "hot" and it is not registered inside the ready
  	 * list, push it inside.
  	 */
  	if (revents & event->events) {
  		spin_lock_irq(&ep->lock);
  		if (!ep_is_linked(&epi->rdllink)) {
  			list_add_tail(&epi->rdllink, &ep->rdllist);
  			ep_pm_stay_awake(epi);
  
  			/* Notify waiting tasks that events are available */
  			if (waitqueue_active(&ep->wq))
  				wake_up_locked(&ep->wq);
  			if (waitqueue_active(&ep->poll_wait))
  				pwake++;
  		}
  		spin_unlock_irq(&ep->lock);
  	}
  
  	/* We have to call this outside the lock */
  	if (pwake)
  		ep_poll_safewake(&ep->poll_wait);
  
  	return 0;
  }
  
  static int ep_send_events_proc(struct eventpoll *ep, struct list_head *head,
  			       void *priv)
  {
  	struct ep_send_events_data *esed = priv;
  	int eventcnt;
  	unsigned int revents;
  	struct epitem *epi;
  	struct epoll_event __user *uevent;
  	struct wakeup_source *ws;
  	poll_table pt;
  
  	init_poll_funcptr(&pt, NULL);
  
  	/*
  	 * We can loop without lock because we are passed a task private list.
  	 * Items cannot vanish during the loop because ep_scan_ready_list() is
  	 * holding "mtx" during this call.
  	 */
  	for (eventcnt = 0, uevent = esed->events;
  	     !list_empty(head) && eventcnt < esed->maxevents;) {
  		epi = list_first_entry(head, struct epitem, rdllink);
  
  		/*
  		 * Activate ep->ws before deactivating epi->ws to prevent
  		 * triggering auto-suspend here (in case we reactive epi->ws
  		 * below).
  		 *
  		 * This could be rearranged to delay the deactivation of epi->ws
  		 * instead, but then epi->ws would temporarily be out of sync
  		 * with ep_is_linked().
  		 */
  		ws = ep_wakeup_source(epi);
  		if (ws) {
  			if (ws->active)
  				__pm_stay_awake(ep->ws);
  			__pm_relax(ws);
  		}
  
  		list_del_init(&epi->rdllink);
  
  		revents = ep_item_poll(epi, &pt);
  
  		/*
  		 * If the event mask intersect the caller-requested one,
  		 * deliver the event to userspace. Again, ep_scan_ready_list()
  		 * is holding "mtx", so no operations coming from userspace
  		 * can change the item.
  		 */
  		if (revents) {
  			if (__put_user(revents, &uevent->events) ||
  			    __put_user(epi->event.data, &uevent->data)) {
  				list_add(&epi->rdllink, head);
  				ep_pm_stay_awake(epi);
  				return eventcnt ? eventcnt : -EFAULT;
  			}
  			eventcnt++;
  			uevent++;
  			if (epi->event.events & EPOLLONESHOT)
  				epi->event.events &= EP_PRIVATE_BITS;
  			else if (!(epi->event.events & EPOLLET)) {
  				/*
  				 * If this file has been added with Level
  				 * Trigger mode, we need to insert back inside
  				 * the ready list, so that the next call to
  				 * epoll_wait() will check again the events
  				 * availability. At this point, no one can insert
  				 * into ep->rdllist besides us. The epoll_ctl()
  				 * callers are locked out by
  				 * ep_scan_ready_list() holding "mtx" and the
  				 * poll callback will queue them in ep->ovflist.
  				 */
  				list_add_tail(&epi->rdllink, &ep->rdllist);
  				ep_pm_stay_awake(epi);
  			}
  		}
  	}
  
  	return eventcnt;
  }
  
  static int ep_send_events(struct eventpoll *ep,
  			  struct epoll_event __user *events, int maxevents)
  {
  	struct ep_send_events_data esed;
  
  	esed.maxevents = maxevents;
  	esed.events = events;
  
  	return ep_scan_ready_list(ep, ep_send_events_proc, &esed, 0, false);
  }
  
  static inline struct timespec ep_set_mstimeout(long ms)
  {
  	struct timespec now, ts = {
  		.tv_sec = ms / MSEC_PER_SEC,
  		.tv_nsec = NSEC_PER_MSEC * (ms % MSEC_PER_SEC),
  	};
  
  	ktime_get_ts(&now);
  	return timespec_add_safe(now, ts);
  }
  
  /**
   * ep_poll - Retrieves ready events, and delivers them to the caller supplied
   *           event buffer.
   *
   * @ep: Pointer to the eventpoll context.
   * @events: Pointer to the userspace buffer where the ready events should be
   *          stored.
   * @maxevents: Size (in terms of number of events) of the caller event buffer.
   * @timeout: Maximum timeout for the ready events fetch operation, in
   *           milliseconds. If the @timeout is zero, the function will not block,
   *           while if the @timeout is less than zero, the function will block
   *           until at least one event has been retrieved (or an error
   *           occurred).
   *
   * Returns: Returns the number of ready events which have been fetched, or an
   *          error code, in case of error.
   */
  static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
  		   int maxevents, long timeout)
  {
  	int res = 0, eavail, timed_out = 0;
  	unsigned long flags;
  	long slack = 0;
  	wait_queue_t wait;
  	ktime_t expires, *to = NULL;
  
  	if (timeout > 0) {
  		struct timespec end_time = ep_set_mstimeout(timeout);
  
  		slack = select_estimate_accuracy(&end_time);
  		to = &expires;
  		*to = timespec_to_ktime(end_time);
  	} else if (timeout == 0) {
  		/*
  		 * Avoid the unnecessary trip to the wait queue loop, if the
  		 * caller specified a non blocking operation.
  		 */
  		timed_out = 1;
  		spin_lock_irqsave(&ep->lock, flags);
  		goto check_events;
  	}
  
  fetch_events:
  	spin_lock_irqsave(&ep->lock, flags);
  
  	if (!ep_events_available(ep)) {
  		/*
  		 * We don't have any available event to return to the caller.
  		 * We need to sleep here, and we will be wake up by
  		 * ep_poll_callback() when events will become available.
  		 */
  		init_waitqueue_entry(&wait, current);
  		__add_wait_queue_exclusive(&ep->wq, &wait);
  
  		for (;;) {
  			/*
  			 * We don't want to sleep if the ep_poll_callback() sends us
  			 * a wakeup in between. That's why we set the task state
  			 * to TASK_INTERRUPTIBLE before doing the checks.
  			 */
  			set_current_state(TASK_INTERRUPTIBLE);
  			if (ep_events_available(ep) || timed_out)
  				break;
  			if (signal_pending(current)) {
  				res = -EINTR;
  				break;
  			}
  
  			spin_unlock_irqrestore(&ep->lock, flags);
  			if (!schedule_hrtimeout_range(to, slack, HRTIMER_MODE_ABS))
  				timed_out = 1;
  
  			spin_lock_irqsave(&ep->lock, flags);
  		}
  
  		__remove_wait_queue(&ep->wq, &wait);
  		__set_current_state(TASK_RUNNING);
  	}
  check_events:
  	/* Is it worth to try to dig for events ? */
  	eavail = ep_events_available(ep);
  
  	spin_unlock_irqrestore(&ep->lock, flags);
  
  	/*
  	 * Try to transfer events to user space. In case we get 0 events and
  	 * there's still timeout left over, we go trying again in search of
  	 * more luck.
  	 */
  	if (!res && eavail &&
  	    !(res = ep_send_events(ep, events, maxevents)) && !timed_out)
  		goto fetch_events;
  
  	return res;
  }
  
  /**
   * ep_loop_check_proc - Callback function to be passed to the @ep_call_nested()
   *                      API, to verify that adding an epoll file inside another
   *                      epoll structure, does not violate the constraints, in
   *                      terms of closed loops, or too deep chains (which can
   *                      result in excessive stack usage).
   *
   * @priv: Pointer to the epoll file to be currently checked.
   * @cookie: Original cookie for this call. This is the top-of-the-chain epoll
   *          data structure pointer.
   * @call_nests: Current dept of the @ep_call_nested() call stack.
   *
   * Returns: Returns zero if adding the epoll @file inside current epoll
   *          structure @ep does not violate the constraints, or -1 otherwise.
   */
  static int ep_loop_check_proc(void *priv, void *cookie, int call_nests)
  {
  	int error = 0;
  	struct file *file = priv;
  	struct eventpoll *ep = file->private_data;
  	struct eventpoll *ep_tovisit;
  	struct rb_node *rbp;
  	struct epitem *epi;
  
  	mutex_lock_nested(&ep->mtx, call_nests + 1);
  	ep->visited = 1;
  	list_add(&ep->visited_list_link, &visited_list);
  	for (rbp = rb_first(&ep->rbr); rbp; rbp = rb_next(rbp)) {
  		epi = rb_entry(rbp, struct epitem, rbn);
  		if (unlikely(is_file_epoll(epi->ffd.file))) {
  			ep_tovisit = epi->ffd.file->private_data;
  			if (ep_tovisit->visited)
  				continue;
  			error = ep_call_nested(&poll_loop_ncalls, EP_MAX_NESTS,
  					ep_loop_check_proc, epi->ffd.file,
  					ep_tovisit, current);
  			if (error != 0)
  				break;
  		} else {
  			/*
  			 * If we've reached a file that is not associated with
  			 * an ep, then we need to check if the newly added
  			 * links are going to add too many wakeup paths. We do
  			 * this by adding it to the tfile_check_list, if it's
  			 * not already there, and calling reverse_path_check()
  			 * during ep_insert().
  			 */
  			if (list_empty(&epi->ffd.file->f_tfile_llink))
  				list_add(&epi->ffd.file->f_tfile_llink,
  					 &tfile_check_list);
  		}
  	}
  	mutex_unlock(&ep->mtx);
  
  	return error;
  }
  
  /**
   * ep_loop_check - Performs a check to verify that adding an epoll file (@file)
   *                 another epoll file (represented by @ep) does not create
   *                 closed loops or too deep chains.
   *
   * @ep: Pointer to the epoll private data structure.
   * @file: Pointer to the epoll file to be checked.
   *
   * Returns: Returns zero if adding the epoll @file inside current epoll
   *          structure @ep does not violate the constraints, or -1 otherwise.
   */
  static int ep_loop_check(struct eventpoll *ep, struct file *file)
  {
  	int ret;
  	struct eventpoll *ep_cur, *ep_next;
  
  	ret = ep_call_nested(&poll_loop_ncalls, EP_MAX_NESTS,
  			      ep_loop_check_proc, file, ep, current);
  	/* clear visited list */
  	list_for_each_entry_safe(ep_cur, ep_next, &visited_list,
  							visited_list_link) {
  		ep_cur->visited = 0;
  		list_del(&ep_cur->visited_list_link);
  	}
  	return ret;
  }
  
  static void clear_tfile_check_list(void)
  {
  	struct file *file;
  
  	/* first clear the tfile_check_list */
  	while (!list_empty(&tfile_check_list)) {
  		file = list_first_entry(&tfile_check_list, struct file,
  					f_tfile_llink);
  		list_del_init(&file->f_tfile_llink);
  	}
  	INIT_LIST_HEAD(&tfile_check_list);
  }
  
  /*
   * Open an eventpoll file descriptor.
   */
  SYSCALL_DEFINE1(epoll_create1, int, flags)
  {
  	int error, fd;
  	struct eventpoll *ep = NULL;
  	struct file *file;
  
  	/* Check the EPOLL_* constant for consistency.  */
  	BUILD_BUG_ON(EPOLL_CLOEXEC != O_CLOEXEC);
  
  	if (flags & ~EPOLL_CLOEXEC)
  		return -EINVAL;
  	/*
  	 * Create the internal data structure ("struct eventpoll").
  	 */
  	error = ep_alloc(&ep);
  	if (error < 0)
  		return error;
  	/*
  	 * Creates all the items needed to setup an eventpoll file. That is,
  	 * a file structure and a free file descriptor.
  	 */
  	fd = get_unused_fd_flags(O_RDWR | (flags & O_CLOEXEC));
  	if (fd < 0) {
  		error = fd;
  		goto out_free_ep;
  	}
  	file = anon_inode_getfile("[eventpoll]", &eventpoll_fops, ep,
  				 O_RDWR | (flags & O_CLOEXEC));
  	if (IS_ERR(file)) {
  		error = PTR_ERR(file);
  		goto out_free_fd;
  	}
  	ep->file = file;
  	fd_install(fd, file);
  	return fd;
  
  out_free_fd:
  	put_unused_fd(fd);
  out_free_ep:
  	ep_free(ep);
  	return error;
  }
  
  SYSCALL_DEFINE1(epoll_create, int, size)
  {
  	if (size <= 0)
  		return -EINVAL;
  
  	return sys_epoll_create1(0);
  }
  
  /*
   * The following function implements the controller interface for
   * the eventpoll file that enables the insertion/removal/change of
   * file descriptors inside the interest set.
   */
  SYSCALL_DEFINE4(epoll_ctl, int, epfd, int, op, int, fd,
  		struct epoll_event __user *, event)
  {
  	int error;
  	int full_check = 0;
  	struct fd f, tf;
  	struct eventpoll *ep;
  	struct epitem *epi;
  	struct epoll_event epds;
  	struct eventpoll *tep = NULL;
  
  	error = -EFAULT;
  	if (ep_op_has_event(op) &&
  	    copy_from_user(&epds, event, sizeof(struct epoll_event)))
  		goto error_return;
  
  	error = -EBADF;
  	f = fdget(epfd);
  	if (!f.file)
  		goto error_return;
  
  	/* Get the "struct file *" for the target file */
  	tf = fdget(fd);
  	if (!tf.file)
  		goto error_fput;
  
  	/* The target file descriptor must support poll */
  	error = -EPERM;
  	if (!tf.file->f_op->poll)
  		goto error_tgt_fput;
  
  	/* Check if EPOLLWAKEUP is allowed */
  	if (ep_op_has_event(op))
  		ep_take_care_of_epollwakeup(&epds);
  
  	/*
  	 * We have to check that the file structure underneath the file descriptor
  	 * the user passed to us _is_ an eventpoll file. And also we do not permit
  	 * adding an epoll file descriptor inside itself.
  	 */
  	error = -EINVAL;
  	if (f.file == tf.file || !is_file_epoll(f.file))
  		goto error_tgt_fput;
  
  	/*
  	 * epoll adds to the wakeup queue at EPOLL_CTL_ADD time only,
  	 * so EPOLLEXCLUSIVE is not allowed for a EPOLL_CTL_MOD operation.
  	 * Also, we do not currently supported nested exclusive wakeups.
  	 */
  	if (epds.events & EPOLLEXCLUSIVE) {
  		if (op == EPOLL_CTL_MOD)
  			goto error_tgt_fput;
  		if (op == EPOLL_CTL_ADD && (is_file_epoll(tf.file) ||
  				(epds.events & ~EPOLLEXCLUSIVE_OK_BITS)))
  			goto error_tgt_fput;
  	}
  
  	/*
  	 * At this point it is safe to assume that the "private_data" contains
  	 * our own data structure.
  	 */
  	ep = f.file->private_data;
  
  	/*
  	 * When we insert an epoll file descriptor, inside another epoll file
  	 * descriptor, there is the change of creating closed loops, which are
  	 * better be handled here, than in more critical paths. While we are
  	 * checking for loops we also determine the list of files reachable
  	 * and hang them on the tfile_check_list, so we can check that we
  	 * haven't created too many possible wakeup paths.
  	 *
  	 * We do not need to take the global 'epumutex' on EPOLL_CTL_ADD when
  	 * the epoll file descriptor is attaching directly to a wakeup source,
  	 * unless the epoll file descriptor is nested. The purpose of taking the
  	 * 'epmutex' on add is to prevent complex toplogies such as loops and
  	 * deep wakeup paths from forming in parallel through multiple
  	 * EPOLL_CTL_ADD operations.
  	 */
  	mutex_lock_nested(&ep->mtx, 0);
  	if (op == EPOLL_CTL_ADD) {
  		if (!list_empty(&f.file->f_ep_links) ||
  						is_file_epoll(tf.file)) {
  			full_check = 1;
  			mutex_unlock(&ep->mtx);
  			mutex_lock(&epmutex);
  			if (is_file_epoll(tf.file)) {
  				error = -ELOOP;
  				if (ep_loop_check(ep, tf.file) != 0) {
  					clear_tfile_check_list();
  					goto error_tgt_fput;
  				}
  			} else
  				list_add(&tf.file->f_tfile_llink,
  							&tfile_check_list);
  			mutex_lock_nested(&ep->mtx, 0);
  			if (is_file_epoll(tf.file)) {
  				tep = tf.file->private_data;
  				mutex_lock_nested(&tep->mtx, 1);
  			}
  		}
  	}
  
  	/*
  	 * Try to lookup the file inside our RB tree, Since we grabbed "mtx"
  	 * above, we can be sure to be able to use the item looked up by
  	 * ep_find() till we release the mutex.
  	 */
  	epi = ep_find(ep, tf.file, fd);
  
  	error = -EINVAL;
  	switch (op) {
  	case EPOLL_CTL_ADD:
  		if (!epi) {
  			epds.events |= POLLERR | POLLHUP;
  			error = ep_insert(ep, &epds, tf.file, fd, full_check);
  		} else
  			error = -EEXIST;
  		if (full_check)
  			clear_tfile_check_list();
  		break;
  	case EPOLL_CTL_DEL:
  		if (epi)
  			error = ep_remove(ep, epi);
  		else
  			error = -ENOENT;
  		break;
  	case EPOLL_CTL_MOD:
  		if (epi) {
  			if (!(epi->event.events & EPOLLEXCLUSIVE)) {
  				epds.events |= POLLERR | POLLHUP;
  				error = ep_modify(ep, epi, &epds);
  			}
  		} else
  			error = -ENOENT;
  		break;
  	}
  	if (tep != NULL)
  		mutex_unlock(&tep->mtx);
  	mutex_unlock(&ep->mtx);
  
  error_tgt_fput:
  	if (full_check)
  		mutex_unlock(&epmutex);
  
  	fdput(tf);
  error_fput:
  	fdput(f);
  error_return:
  
  	return error;
  }
  
  /*
   * Implement the event wait interface for the eventpoll file. It is the kernel
   * part of the user space epoll_wait(2).
   */
  SYSCALL_DEFINE4(epoll_wait, int, epfd, struct epoll_event __user *, events,
  		int, maxevents, int, timeout)
  {
  	int error;
  	struct fd f;
  	struct eventpoll *ep;
  
  	/* The maximum number of event must be greater than zero */
  	if (maxevents <= 0 || maxevents > EP_MAX_EVENTS)
  		return -EINVAL;
  
  	/* Verify that the area passed by the user is writeable */
  	if (!access_ok(VERIFY_WRITE, events, maxevents * sizeof(struct epoll_event)))
  		return -EFAULT;
  
  	/* Get the "struct file *" for the eventpoll file */
  	f = fdget(epfd);
  	if (!f.file)
  		return -EBADF;
  
  	/*
  	 * We have to check that the file structure underneath the fd
  	 * the user passed to us _is_ an eventpoll file.
  	 */
  	error = -EINVAL;
  	if (!is_file_epoll(f.file))
  		goto error_fput;
  
  	/*
  	 * At this point it is safe to assume that the "private_data" contains
  	 * our own data structure.
  	 */
  	ep = f.file->private_data;
  
  	/* Time to fish for events ... */
  	error = ep_poll(ep, events, maxevents, timeout);
  
  error_fput:
  	fdput(f);
  	return error;
  }
  
  /*
   * Implement the event wait interface for the eventpoll file. It is the kernel
   * part of the user space epoll_pwait(2).
   */
  SYSCALL_DEFINE6(epoll_pwait, int, epfd, struct epoll_event __user *, events,
  		int, maxevents, int, timeout, const sigset_t __user *, sigmask,
  		size_t, sigsetsize)
  {
  	int error;
  	sigset_t ksigmask, sigsaved;
  
  	/*
  	 * If the caller wants a certain signal mask to be set during the wait,
  	 * we apply it here.
  	 */
  	if (sigmask) {
  		if (sigsetsize != sizeof(sigset_t))
  			return -EINVAL;
  		if (copy_from_user(&ksigmask, sigmask, sizeof(ksigmask)))
  			return -EFAULT;
  		sigsaved = current->blocked;
  		set_current_blocked(&ksigmask);
  	}
  
  	error = sys_epoll_wait(epfd, events, maxevents, timeout);
  
  	/*
  	 * If we changed the signal mask, we need to restore the original one.
  	 * In case we've got a signal while waiting, we do not restore the
  	 * signal mask yet, and we allow do_signal() to deliver the signal on
  	 * the way back to userspace, before the signal mask is restored.
  	 */
  	if (sigmask) {
  		if (error == -EINTR) {
  			memcpy(&current->saved_sigmask, &sigsaved,
  			       sizeof(sigsaved));
  			set_restore_sigmask();
  		} else
  			set_current_blocked(&sigsaved);
  	}
  
  	return error;
  }
  
  #ifdef CONFIG_COMPAT
  COMPAT_SYSCALL_DEFINE6(epoll_pwait, int, epfd,
  			struct epoll_event __user *, events,
  			int, maxevents, int, timeout,
  			const compat_sigset_t __user *, sigmask,
  			compat_size_t, sigsetsize)
  {
  	long err;
  	compat_sigset_t csigmask;
  	sigset_t ksigmask, sigsaved;
  
  	/*
  	 * If the caller wants a certain signal mask to be set during the wait,
  	 * we apply it here.
  	 */
  	if (sigmask) {
  		if (sigsetsize != sizeof(compat_sigset_t))
  			return -EINVAL;
  		if (copy_from_user(&csigmask, sigmask, sizeof(csigmask)))
  			return -EFAULT;
  		sigset_from_compat(&ksigmask, &csigmask);
  		sigsaved = current->blocked;
  		set_current_blocked(&ksigmask);
  	}
  
  	err = sys_epoll_wait(epfd, events, maxevents, timeout);
  
  	/*
  	 * If we changed the signal mask, we need to restore the original one.
  	 * In case we've got a signal while waiting, we do not restore the
  	 * signal mask yet, and we allow do_signal() to deliver the signal on
  	 * the way back to userspace, before the signal mask is restored.
  	 */
  	if (sigmask) {
  		if (err == -EINTR) {
  			memcpy(&current->saved_sigmask, &sigsaved,
  			       sizeof(sigsaved));
  			set_restore_sigmask();
  		} else
  			set_current_blocked(&sigsaved);
  	}
  
  	return err;
  }
  #endif
  
  static int __init eventpoll_init(void)
  {
  	struct sysinfo si;
  
  	si_meminfo(&si);
  	/*
  	 * Allows top 4% of lomem to be allocated for epoll watches (per user).
  	 */
  	max_user_watches = (((si.totalram - si.totalhigh) / 25) << PAGE_SHIFT) /
  		EP_ITEM_COST;
  	BUG_ON(max_user_watches < 0);
  
  	/*
  	 * Initialize the structure used to perform epoll file descriptor
  	 * inclusion loops checks.
  	 */
  	ep_nested_calls_init(&poll_loop_ncalls);
  
  	/* Initialize the structure used to perform safe poll wait head wake ups */
  	ep_nested_calls_init(&poll_safewake_ncalls);
  
  	/* Initialize the structure used to perform file's f_op->poll() calls */
  	ep_nested_calls_init(&poll_readywalk_ncalls);
  
  	/*
  	 * We can have many thousands of epitems, so prevent this from
  	 * using an extra cache line on 64-bit (and smaller) CPUs
  	 */
  	BUILD_BUG_ON(sizeof(void *) <= 8 && sizeof(struct epitem) > 128);
  
  	/* Allocates slab cache used to allocate "struct epitem" items */
  	epi_cache = kmem_cache_create("eventpoll_epi", sizeof(struct epitem),
  			0, SLAB_HWCACHE_ALIGN | SLAB_PANIC, NULL);
  
  	/* Allocates slab cache used to allocate "struct eppoll_entry" */
  	pwq_cache = kmem_cache_create("eventpoll_pwq",
  			sizeof(struct eppoll_entry), 0, SLAB_PANIC, NULL);
  
  	return 0;
  }
  fs_initcall(eventpoll_init);
  