text: /* rwsem.c: R/W semaphores: contention handling functions
   *
   * Written by David Howells (dhowells@redhat.com).
   * Derived from arch/i386/kernel/semaphore.c
   *
   * Writer lock-stealing by Alex Shi <alex.shi@intel.com>
   * and Michel Lespinasse <walken@google.com>
   *
   * Optimistic spinning by Tim Chen <tim.c.chen@intel.com>
   * and Davidlohr Bueso <davidlohr@hp.com>. Based on mutexes.
   */
  #include <linux/rwsem.h>
  #include <linux/sched.h>
  #include <linux/init.h>
  #include <linux/export.h>
  #include <linux/sched/rt.h>
  #include <linux/osq_lock.h>
  
  #include "rwsem.h"
  
  /*
   * Guide to the rw_semaphore's count field for common values.
   * (32-bit case illustrated, similar for 64-bit)
   *
   * 0x0000000X	(1) X readers active or attempting lock, no writer waiting
   *		    X = #active_readers + #readers attempting to lock
   *		    (X*ACTIVE_BIAS)
   *
   * 0x00000000	rwsem is unlocked, and no one is waiting for the lock or
   *		attempting to read lock or write lock.
   *
   * 0xffff000X	(1) X readers active or attempting lock, with waiters for lock
   *		    X = #active readers + # readers attempting lock
   *		    (X*ACTIVE_BIAS + WAITING_BIAS)
   *		(2) 1 writer attempting lock, no waiters for lock
   *		    X-1 = #active readers + #readers attempting lock
   *		    ((X-1)*ACTIVE_BIAS + ACTIVE_WRITE_BIAS)
   *		(3) 1 writer active, no waiters for lock
   *		    X-1 = #active readers + #readers attempting lock
   *		    ((X-1)*ACTIVE_BIAS + ACTIVE_WRITE_BIAS)
   *
   * 0xffff0001	(1) 1 reader active or attempting lock, waiters for lock
   *		    (WAITING_BIAS + ACTIVE_BIAS)
   *		(2) 1 writer active or attempting lock, no waiters for lock
   *		    (ACTIVE_WRITE_BIAS)
   *
   * 0xffff0000	(1) There are writers or readers queued but none active
   *		    or in the process of attempting lock.
   *		    (WAITING_BIAS)
   *		Note: writer can attempt to steal lock for this count by adding
   *		ACTIVE_WRITE_BIAS in cmpxchg and checking the old count
   *
   * 0xfffe0001	(1) 1 writer active, or attempting lock. Waiters on queue.
   *		    (ACTIVE_WRITE_BIAS + WAITING_BIAS)
   *
   * Note: Readers attempt to lock by adding ACTIVE_BIAS in down_read and checking
   *	 the count becomes more than 0 for successful lock acquisition,
   *	 i.e. the case where there are only readers or nobody has lock.
   *	 (1st and 2nd case above).
   *
   *	 Writers attempt to lock by adding ACTIVE_WRITE_BIAS in down_write and
   *	 checking the count becomes ACTIVE_WRITE_BIAS for successful lock
   *	 acquisition (i.e. nobody else has lock or attempts lock).  If
   *	 unsuccessful, in rwsem_down_write_failed, we'll check to see if there
   *	 are only waiters but none active (5th case above), and attempt to
   *	 steal the lock.
   *
   */
  
  /*
   * Initialize an rwsem:
   */
  void __init_rwsem(struct rw_semaphore *sem, const char *name,
  		  struct lock_class_key *key)
  {
  #ifdef CONFIG_DEBUG_LOCK_ALLOC
  	/*
  	 * Make sure we are not reinitializing a held semaphore:
  	 */
  	debug_check_no_locks_freed((void *)sem, sizeof(*sem));
  	lockdep_init_map(&sem->dep_map, name, key, 0);
  #endif
  	sem->count = RWSEM_UNLOCKED_VALUE;
  	raw_spin_lock_init(&sem->wait_lock);
  	INIT_LIST_HEAD(&sem->wait_list);
  #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
  	sem->owner = NULL;
  	osq_lock_init(&sem->osq);
  #endif
  }
  
  EXPORT_SYMBOL(__init_rwsem);
  
  enum rwsem_waiter_type {
  	RWSEM_WAITING_FOR_WRITE,
  	RWSEM_WAITING_FOR_READ
  };
  
  struct rwsem_waiter {
  	struct list_head list;
  	struct task_struct *task;
  	enum rwsem_waiter_type type;
  };
  
  enum rwsem_wake_type {
  	RWSEM_WAKE_ANY,		/* Wake whatever's at head of wait list */
  	RWSEM_WAKE_READERS,	/* Wake readers only */
  	RWSEM_WAKE_READ_OWNED	/* Waker thread holds the read lock */
  };
  
  /*
   * handle the lock release when processes blocked on it that can now run
   * - if we come here from up_xxxx(), then:
   *   - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed)
   *   - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so)
   * - there must be someone on the queue
   * - the spinlock must be held by the caller
   * - woken process blocks are discarded from the list after having task zeroed
   * - writers are only woken if downgrading is false
   */
  static struct rw_semaphore *
  __rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type)
  {
  	struct rwsem_waiter *waiter;
  	struct task_struct *tsk;
  	struct list_head *next;
  	long oldcount, woken, loop, adjustment;
  
  	waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
  	if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
  		if (wake_type == RWSEM_WAKE_ANY)
  			/* Wake writer at the front of the queue, but do not
  			 * grant it the lock yet as we want other writers
  			 * to be able to steal it.  Readers, on the other hand,
  			 * will block as they will notice the queued writer.
  			 */
  			wake_up_process(waiter->task);
  		goto out;
  	}
  
  	/* Writers might steal the lock before we grant it to the next reader.
  	 * We prefer to do the first reader grant before counting readers
  	 * so we can bail out early if a writer stole the lock.
  	 */
  	adjustment = 0;
  	if (wake_type != RWSEM_WAKE_READ_OWNED) {
  		adjustment = RWSEM_ACTIVE_READ_BIAS;
   try_reader_grant:
  		oldcount = rwsem_atomic_update(adjustment, sem) - adjustment;
  		if (unlikely(oldcount < RWSEM_WAITING_BIAS)) {
  			/* A writer stole the lock. Undo our reader grant. */
  			if (rwsem_atomic_update(-adjustment, sem) &
  						RWSEM_ACTIVE_MASK)
  				goto out;
  			/* Last active locker left. Retry waking readers. */
  			goto try_reader_grant;
  		}
  	}
  
  	/* Grant an infinite number of read locks to the readers at the front
  	 * of the queue.  Note we increment the 'active part' of the count by
  	 * the number of readers before waking any processes up.
  	 */
  	woken = 0;
  	do {
  		woken++;
  
  		if (waiter->list.next == &sem->wait_list)
  			break;
  
  		waiter = list_entry(waiter->list.next,
  					struct rwsem_waiter, list);
  
  	} while (waiter->type != RWSEM_WAITING_FOR_WRITE);
  
  	adjustment = woken * RWSEM_ACTIVE_READ_BIAS - adjustment;
  	if (waiter->type != RWSEM_WAITING_FOR_WRITE)
  		/* hit end of list above */
  		adjustment -= RWSEM_WAITING_BIAS;
  
  	if (adjustment)
  		rwsem_atomic_add(adjustment, sem);
  
  	next = sem->wait_list.next;
  	loop = woken;
  	do {
  		waiter = list_entry(next, struct rwsem_waiter, list);
  		next = waiter->list.next;
  		tsk = waiter->task;
  		/*
  		 * Make sure we do not wakeup the next reader before
  		 * setting the nil condition to grant the next reader;
  		 * otherwise we could miss the wakeup on the other
  		 * side and end up sleeping again. See the pairing
  		 * in rwsem_down_read_failed().
  		 */
  		smp_mb();
  		waiter->task = NULL;
  		wake_up_process(tsk);
  		put_task_struct(tsk);
  	} while (--loop);
  
  	sem->wait_list.next = next;
  	next->prev = &sem->wait_list;
  
   out:
  	return sem;
  }
  
  /*
   * Wait for the read lock to be granted
   */
  __visible
  struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
  {
  	long count, adjustment = -RWSEM_ACTIVE_READ_BIAS;
  	struct rwsem_waiter waiter;
  	struct task_struct *tsk = current;
  
  	/* set up my own style of waitqueue */
  	waiter.task = tsk;
  	waiter.type = RWSEM_WAITING_FOR_READ;
  	get_task_struct(tsk);
  
  	raw_spin_lock_irq(&sem->wait_lock);
  	if (list_empty(&sem->wait_list))
  		adjustment += RWSEM_WAITING_BIAS;
  	list_add_tail(&waiter.list, &sem->wait_list);
  
  	/* we're now waiting on the lock, but no longer actively locking */
  	count = rwsem_atomic_update(adjustment, sem);
  
  	/* If there are no active locks, wake the front queued process(es).
  	 *
  	 * If there are no writers and we are first in the queue,
  	 * wake our own waiter to join the existing active readers !
  	 */
  	if (count == RWSEM_WAITING_BIAS ||
  	    (count > RWSEM_WAITING_BIAS &&
  	     adjustment != -RWSEM_ACTIVE_READ_BIAS))
  		sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY);
  
  	raw_spin_unlock_irq(&sem->wait_lock);
  
  	/* wait to be given the lock */
  	while (true) {
  		set_task_state(tsk, TASK_UNINTERRUPTIBLE);
  		if (!waiter.task)
  			break;
  		schedule();
  	}
  
  	__set_task_state(tsk, TASK_RUNNING);
  	return sem;
  }
  EXPORT_SYMBOL(rwsem_down_read_failed);
  
  static inline bool rwsem_try_write_lock(long count, struct rw_semaphore *sem)
  {
  	/*
  	 * Try acquiring the write lock. Check count first in order
  	 * to reduce unnecessary expensive cmpxchg() operations.
  	 */
  	if (count == RWSEM_WAITING_BIAS &&
  	    cmpxchg(&sem->count, RWSEM_WAITING_BIAS,
  		    RWSEM_ACTIVE_WRITE_BIAS) == RWSEM_WAITING_BIAS) {
  		if (!list_is_singular(&sem->wait_list))
  			rwsem_atomic_update(RWSEM_WAITING_BIAS, sem);
  		rwsem_set_owner(sem);
  		return true;
  	}
  
  	return false;
  }
  
  #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
  /*
   * Try to acquire write lock before the writer has been put on wait queue.
   */
  static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
  {
  	long old, count = READ_ONCE(sem->count);
  
  	while (true) {
  		if (!(count == 0 || count == RWSEM_WAITING_BIAS))
  			return false;
  
  		old = cmpxchg(&sem->count, count, count + RWSEM_ACTIVE_WRITE_BIAS);
  		if (old == count) {
  			rwsem_set_owner(sem);
  			return true;
  		}
  
  		count = old;
  	}
  }
  
  static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
  {
  	struct task_struct *owner;
  	bool ret = true;
  
  	if (need_resched())
  		return false;
  
  	rcu_read_lock();
  	owner = READ_ONCE(sem->owner);
  	if (!owner) {
  		long count = READ_ONCE(sem->count);
  		/*
  		 * If sem->owner is not set, yet we have just recently entered the
  		 * slowpath with the lock being active, then there is a possibility
  		 * reader(s) may have the lock. To be safe, bail spinning in these
  		 * situations.
  		 */
  		if (count & RWSEM_ACTIVE_MASK)
  			ret = false;
  		goto done;
  	}
  
  	ret = owner->on_cpu;
  done:
  	rcu_read_unlock();
  	return ret;
  }
  
  static noinline
  bool rwsem_spin_on_owner(struct rw_semaphore *sem, struct task_struct *owner)
  {
  	long count;
  
  	rcu_read_lock();
  	while (sem->owner == owner) {
  		/*
  		 * Ensure we emit the owner->on_cpu, dereference _after_
  		 * checking sem->owner still matches owner, if that fails,
  		 * owner might point to free()d memory, if it still matches,
  		 * the rcu_read_lock() ensures the memory stays valid.
  		 */
  		barrier();
  
  		/* abort spinning when need_resched or owner is not running */
  		if (!owner->on_cpu || need_resched()) {
  			rcu_read_unlock();
  			return false;
  		}
  
  		cpu_relax_lowlatency();
  	}
  	rcu_read_unlock();
  
  	if (READ_ONCE(sem->owner))
  		return true; /* new owner, continue spinning */
  
  	/*
  	 * When the owner is not set, the lock could be free or
  	 * held by readers. Check the counter to verify the
  	 * state.
  	 */
  	count = READ_ONCE(sem->count);
  	return (count == 0 || count == RWSEM_WAITING_BIAS);
  }
  
  static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
  {
  	struct task_struct *owner;
  	bool taken = false;
  
  	preempt_disable();
  
  	/* sem->wait_lock should not be held when doing optimistic spinning */
  	if (!rwsem_can_spin_on_owner(sem))
  		goto done;
  
  	if (!osq_lock(&sem->osq))
  		goto done;
  
  	while (true) {
  		owner = READ_ONCE(sem->owner);
  		if (owner && !rwsem_spin_on_owner(sem, owner))
  			break;
  
  		/* wait_lock will be acquired if write_lock is obtained */
  		if (rwsem_try_write_lock_unqueued(sem)) {
  			taken = true;
  			break;
  		}
  
  		/*
  		 * When there's no owner, we might have preempted between the
  		 * owner acquiring the lock and setting the owner field. If
  		 * we're an RT task that will live-lock because we won't let
  		 * the owner complete.
  		 */
  		if (!owner && (need_resched() || rt_task(current)))
  			break;
  
  		/*
  		 * The cpu_relax() call is a compiler barrier which forces
  		 * everything in this loop to be re-loaded. We don't need
  		 * memory barriers as we'll eventually observe the right
  		 * values at the cost of a few extra spins.
  		 */
  		cpu_relax_lowlatency();
  	}
  	osq_unlock(&sem->osq);
  done:
  	preempt_enable();
  	return taken;
  }
  
  /*
   * Return true if the rwsem has active spinner
   */
  static inline bool rwsem_has_spinner(struct rw_semaphore *sem)
  {
  	return osq_is_locked(&sem->osq);
  }
  
  #else
  static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
  {
  	return false;
  }
  
  static inline bool rwsem_has_spinner(struct rw_semaphore *sem)
  {
  	return false;
  }
  #endif
  
  /*
   * Wait until we successfully acquire the write lock
   */
  __visible
  struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem)
  {
  	long count;
  	bool waiting = true; /* any queued threads before us */
  	struct rwsem_waiter waiter;
  
  	/* undo write bias from down_write operation, stop active locking */
  	count = rwsem_atomic_update(-RWSEM_ACTIVE_WRITE_BIAS, sem);
  
  	/* do optimistic spinning and steal lock if possible */
  	if (rwsem_optimistic_spin(sem))
  		return sem;
  
  	/*
  	 * Optimistic spinning failed, proceed to the slowpath
  	 * and block until we can acquire the sem.
  	 */
  	waiter.task = current;
  	waiter.type = RWSEM_WAITING_FOR_WRITE;
  
  	raw_spin_lock_irq(&sem->wait_lock);
  
  	/* account for this before adding a new element to the list */
  	if (list_empty(&sem->wait_list))
  		waiting = false;
  
  	list_add_tail(&waiter.list, &sem->wait_list);
  
  	/* we're now waiting on the lock, but no longer actively locking */
  	if (waiting) {
  		count = READ_ONCE(sem->count);
  
  		/*
  		 * If there were already threads queued before us and there are
  		 * no active writers, the lock must be read owned; so we try to
  		 * wake any read locks that were queued ahead of us.
  		 */
  		if (count > RWSEM_WAITING_BIAS)
  			sem = __rwsem_do_wake(sem, RWSEM_WAKE_READERS);
  
  	} else
  		count = rwsem_atomic_update(RWSEM_WAITING_BIAS, sem);
  
  	/* wait until we successfully acquire the lock */
  	set_current_state(TASK_UNINTERRUPTIBLE);
  	while (true) {
  		if (rwsem_try_write_lock(count, sem))
  			break;
  		raw_spin_unlock_irq(&sem->wait_lock);
  
  		/* Block until there are no active lockers. */
  		do {
  			schedule();
  			set_current_state(TASK_UNINTERRUPTIBLE);
  		} while ((count = sem->count) & RWSEM_ACTIVE_MASK);
  
  		raw_spin_lock_irq(&sem->wait_lock);
  	}
  	__set_current_state(TASK_RUNNING);
  
  	list_del(&waiter.list);
  	raw_spin_unlock_irq(&sem->wait_lock);
  
  	return sem;
  }
  EXPORT_SYMBOL(rwsem_down_write_failed);
  
  /*
   * handle waking up a waiter on the semaphore
   * - up_read/up_write has decremented the active part of count if we come here
   */
  __visible
  struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
  {
  	unsigned long flags;
  
  	/*
  	 * If a spinner is present, it is not necessary to do the wakeup.
  	 * Try to do wakeup only if the trylock succeeds to minimize
  	 * spinlock contention which may introduce too much delay in the
  	 * unlock operation.
  	 *
  	 *    spinning writer		up_write/up_read caller
  	 *    ---------------		-----------------------
  	 * [S]   osq_unlock()		[L]   osq
  	 *	 MB			      RMB
  	 * [RmW] rwsem_try_write_lock() [RmW] spin_trylock(wait_lock)
  	 *
  	 * Here, it is important to make sure that there won't be a missed
  	 * wakeup while the rwsem is free and the only spinning writer goes
  	 * to sleep without taking the rwsem. Even when the spinning writer
  	 * is just going to break out of the waiting loop, it will still do
  	 * a trylock in rwsem_down_write_failed() before sleeping. IOW, if
  	 * rwsem_has_spinner() is true, it will guarantee at least one
  	 * trylock attempt on the rwsem later on.
  	 */
  	if (rwsem_has_spinner(sem)) {
  		/*
  		 * The smp_rmb() here is to make sure that the spinner
  		 * state is consulted before reading the wait_lock.
  		 */
  		smp_rmb();
  		if (!raw_spin_trylock_irqsave(&sem->wait_lock, flags))
  			return sem;
  		goto locked;
  	}
  	raw_spin_lock_irqsave(&sem->wait_lock, flags);
  locked:
  
  	/* do nothing if list empty */
  	if (!list_empty(&sem->wait_list))
  		sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY);
  
  	raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
  
  	return sem;
  }
  EXPORT_SYMBOL(rwsem_wake);
  
  /*
   * downgrade a write lock into a read lock
   * - caller incremented waiting part of count and discovered it still negative
   * - just wake up any readers at the front of the queue
   */
  __visible
  struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem)
  {
  	unsigned long flags;
  
  	raw_spin_lock_irqsave(&sem->wait_lock, flags);
  
  	/* do nothing if list empty */
  	if (!list_empty(&sem->wait_list))
  		sem = __rwsem_do_wake(sem, RWSEM_WAKE_READ_OWNED);
  
  	raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
  
  	return sem;
  }
  EXPORT_SYMBOL(rwsem_downgrade_wake);
  