text: /*
   * Read-Copy Update mechanism for mutual exclusion
   *
   * 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.
   *
   * This program is distributed in the hope that it will be useful,
   * but WITHOUT ANY WARRANTY; without even the implied warranty of
   * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   * GNU General Public License for more details.
   *
   * You should have received a copy of the GNU General Public License
   * along with this program; if not, you can access it online at
   * http://www.gnu.org/licenses/gpl-2.0.html.
   *
   * Copyright IBM Corporation, 2001
   *
   * Author: Dipankar Sarma <dipankar@in.ibm.com>
   *
   * Based on the original work by Paul McKenney <paulmck@us.ibm.com>
   * and inputs from Rusty Russell, Andrea Arcangeli and Andi Kleen.
   * Papers:
   * http://www.rdrop.com/users/paulmck/paper/rclockpdcsproof.pdf
   * http://lse.sourceforge.net/locking/rclock_OLS.2001.05.01c.sc.pdf (OLS2001)
   *
   * For detailed explanation of Read-Copy Update mechanism see -
   *		http://lse.sourceforge.net/locking/rcupdate.html
   *
   */
  
  #ifndef __LINUX_RCUPDATE_H
  #define __LINUX_RCUPDATE_H
  
  #include <linux/types.h>
  #include <linux/cache.h>
  #include <linux/spinlock.h>
  #include <linux/threads.h>
  #include <linux/cpumask.h>
  #include <linux/seqlock.h>
  #include <linux/lockdep.h>
  #include <linux/completion.h>
  #include <linux/debugobjects.h>
  #include <linux/bug.h>
  #include <linux/compiler.h>
  #include <asm/barrier.h>
  
  extern int rcu_expedited; /* for sysctl */
  
  enum rcutorture_type {
  	RCU_FLAVOR,
  	RCU_BH_FLAVOR,
  	RCU_SCHED_FLAVOR,
  	RCU_TASKS_FLAVOR,
  	SRCU_FLAVOR,
  	INVALID_RCU_FLAVOR
  };
  
  #if defined(CONFIG_TREE_RCU) || defined(CONFIG_PREEMPT_RCU)
  void rcutorture_get_gp_data(enum rcutorture_type test_type, int *flags,
  			    unsigned long *gpnum, unsigned long *completed);
  void rcutorture_record_test_transition(void);
  void rcutorture_record_progress(unsigned long vernum);
  void do_trace_rcu_torture_read(const char *rcutorturename,
  			       struct rcu_head *rhp,
  			       unsigned long secs,
  			       unsigned long c_old,
  			       unsigned long c);
  #else
  static inline void rcutorture_get_gp_data(enum rcutorture_type test_type,
  					  int *flags,
  					  unsigned long *gpnum,
  					  unsigned long *completed)
  {
  	*flags = 0;
  	*gpnum = 0;
  	*completed = 0;
  }
  static inline void rcutorture_record_test_transition(void)
  {
  }
  static inline void rcutorture_record_progress(unsigned long vernum)
  {
  }
  #ifdef CONFIG_RCU_TRACE
  void do_trace_rcu_torture_read(const char *rcutorturename,
  			       struct rcu_head *rhp,
  			       unsigned long secs,
  			       unsigned long c_old,
  			       unsigned long c);
  #else
  #define do_trace_rcu_torture_read(rcutorturename, rhp, secs, c_old, c) \
  	do { } while (0)
  #endif
  #endif
  
  #define UINT_CMP_GE(a, b)	(UINT_MAX / 2 >= (a) - (b))
  #define UINT_CMP_LT(a, b)	(UINT_MAX / 2 < (a) - (b))
  #define ULONG_CMP_GE(a, b)	(ULONG_MAX / 2 >= (a) - (b))
  #define ULONG_CMP_LT(a, b)	(ULONG_MAX / 2 < (a) - (b))
  #define ulong2long(a)		(*(long *)(&(a)))
  
  /* Exported common interfaces */
  
  #ifdef CONFIG_PREEMPT_RCU
  
  /**
   * call_rcu() - Queue an RCU callback for invocation after a grace period.
   * @head: structure to be used for queueing the RCU updates.
   * @func: actual callback function to be invoked after the grace period
   *
   * The callback function will be invoked some time after a full grace
   * period elapses, in other words after all pre-existing RCU read-side
   * critical sections have completed.  However, the callback function
   * might well execute concurrently with RCU read-side critical sections
   * that started after call_rcu() was invoked.  RCU read-side critical
   * sections are delimited by rcu_read_lock() and rcu_read_unlock(),
   * and may be nested.
   *
   * Note that all CPUs must agree that the grace period extended beyond
   * all pre-existing RCU read-side critical section.  On systems with more
   * than one CPU, this means that when "func()" is invoked, each CPU is
   * guaranteed to have executed a full memory barrier since the end of its
   * last RCU read-side critical section whose beginning preceded the call
   * to call_rcu().  It also means that each CPU executing an RCU read-side
   * critical section that continues beyond the start of "func()" must have
   * executed a memory barrier after the call_rcu() but before the beginning
   * of that RCU read-side critical section.  Note that these guarantees
   * include CPUs that are offline, idle, or executing in user mode, as
   * well as CPUs that are executing in the kernel.
   *
   * Furthermore, if CPU A invoked call_rcu() and CPU B invoked the
   * resulting RCU callback function "func()", then both CPU A and CPU B are
   * guaranteed to execute a full memory barrier during the time interval
   * between the call to call_rcu() and the invocation of "func()" -- even
   * if CPU A and CPU B are the same CPU (but again only if the system has
   * more than one CPU).
   */
  void call_rcu(struct rcu_head *head,
  	      void (*func)(struct rcu_head *head));
  
  #else /* #ifdef CONFIG_PREEMPT_RCU */
  
  /* In classic RCU, call_rcu() is just call_rcu_sched(). */
  #define	call_rcu	call_rcu_sched
  
  #endif /* #else #ifdef CONFIG_PREEMPT_RCU */
  
  /**
   * call_rcu_bh() - Queue an RCU for invocation after a quicker grace period.
   * @head: structure to be used for queueing the RCU updates.
   * @func: actual callback function to be invoked after the grace period
   *
   * The callback function will be invoked some time after a full grace
   * period elapses, in other words after all currently executing RCU
   * read-side critical sections have completed. call_rcu_bh() assumes
   * that the read-side critical sections end on completion of a softirq
   * handler. This means that read-side critical sections in process
   * context must not be interrupted by softirqs. This interface is to be
   * used when most of the read-side critical sections are in softirq context.
   * RCU read-side critical sections are delimited by :
   *  - rcu_read_lock() and  rcu_read_unlock(), if in interrupt context.
   *  OR
   *  - rcu_read_lock_bh() and rcu_read_unlock_bh(), if in process context.
   *  These may be nested.
   *
   * See the description of call_rcu() for more detailed information on
   * memory ordering guarantees.
   */
  void call_rcu_bh(struct rcu_head *head,
  		 void (*func)(struct rcu_head *head));
  
  /**
   * call_rcu_sched() - Queue an RCU for invocation after sched grace period.
   * @head: structure to be used for queueing the RCU updates.
   * @func: actual callback function to be invoked after the grace period
   *
   * The callback function will be invoked some time after a full grace
   * period elapses, in other words after all currently executing RCU
   * read-side critical sections have completed. call_rcu_sched() assumes
   * that the read-side critical sections end on enabling of preemption
   * or on voluntary preemption.
   * RCU read-side critical sections are delimited by :
   *  - rcu_read_lock_sched() and  rcu_read_unlock_sched(),
   *  OR
   *  anything that disables preemption.
   *  These may be nested.
   *
   * See the description of call_rcu() for more detailed information on
   * memory ordering guarantees.
   */
  void call_rcu_sched(struct rcu_head *head,
  		    void (*func)(struct rcu_head *rcu));
  
  void synchronize_sched(void);
  
  /**
   * call_rcu_tasks() - Queue an RCU for invocation task-based grace period
   * @head: structure to be used for queueing the RCU updates.
   * @func: actual callback function to be invoked after the grace period
   *
   * The callback function will be invoked some time after a full grace
   * period elapses, in other words after all currently executing RCU
   * read-side critical sections have completed. call_rcu_tasks() assumes
   * that the read-side critical sections end at a voluntary context
   * switch (not a preemption!), entry into idle, or transition to usermode
   * execution.  As such, there are no read-side primitives analogous to
   * rcu_read_lock() and rcu_read_unlock() because this primitive is intended
   * to determine that all tasks have passed through a safe state, not so
   * much for data-strcuture synchronization.
   *
   * See the description of call_rcu() for more detailed information on
   * memory ordering guarantees.
   */
  void call_rcu_tasks(struct rcu_head *head, void (*func)(struct rcu_head *head));
  void synchronize_rcu_tasks(void);
  void rcu_barrier_tasks(void);
  
  #ifdef CONFIG_PREEMPT_RCU
  
  void __rcu_read_lock(void);
  void __rcu_read_unlock(void);
  void rcu_read_unlock_special(struct task_struct *t);
  void synchronize_rcu(void);
  
  /*
   * Defined as a macro as it is a very low level header included from
   * areas that don't even know about current.  This gives the rcu_read_lock()
   * nesting depth, but makes sense only if CONFIG_PREEMPT_RCU -- in other
   * types of kernel builds, the rcu_read_lock() nesting depth is unknowable.
   */
  #define rcu_preempt_depth() (current->rcu_read_lock_nesting)
  
  #else /* #ifdef CONFIG_PREEMPT_RCU */
  
  static inline void __rcu_read_lock(void)
  {
  	preempt_disable();
  }
  
  static inline void __rcu_read_unlock(void)
  {
  	preempt_enable();
  }
  
  static inline void synchronize_rcu(void)
  {
  	synchronize_sched();
  }
  
  static inline int rcu_preempt_depth(void)
  {
  	return 0;
  }
  
  #endif /* #else #ifdef CONFIG_PREEMPT_RCU */
  
  /* Internal to kernel */
  void rcu_init(void);
  void rcu_sched_qs(void);
  void rcu_bh_qs(void);
  void rcu_check_callbacks(int user);
  struct notifier_block;
  void rcu_idle_enter(void);
  void rcu_idle_exit(void);
  void rcu_irq_enter(void);
  void rcu_irq_exit(void);
  
  #ifdef CONFIG_RCU_STALL_COMMON
  void rcu_sysrq_start(void);
  void rcu_sysrq_end(void);
  #else /* #ifdef CONFIG_RCU_STALL_COMMON */
  static inline void rcu_sysrq_start(void)
  {
  }
  static inline void rcu_sysrq_end(void)
  {
  }
  #endif /* #else #ifdef CONFIG_RCU_STALL_COMMON */
  
  #ifdef CONFIG_RCU_USER_QS
  void rcu_user_enter(void);
  void rcu_user_exit(void);
  #else
  static inline void rcu_user_enter(void) { }
  static inline void rcu_user_exit(void) { }
  static inline void rcu_user_hooks_switch(struct task_struct *prev,
  					 struct task_struct *next) { }
  #endif /* CONFIG_RCU_USER_QS */
  
  #ifdef CONFIG_RCU_NOCB_CPU
  void rcu_init_nohz(void);
  #else /* #ifdef CONFIG_RCU_NOCB_CPU */
  static inline void rcu_init_nohz(void)
  {
  }
  #endif /* #else #ifdef CONFIG_RCU_NOCB_CPU */
  
  /**
   * RCU_NONIDLE - Indicate idle-loop code that needs RCU readers
   * @a: Code that RCU needs to pay attention to.
   *
   * RCU, RCU-bh, and RCU-sched read-side critical sections are forbidden
   * in the inner idle loop, that is, between the rcu_idle_enter() and
   * the rcu_idle_exit() -- RCU will happily ignore any such read-side
   * critical sections.  However, things like powertop need tracepoints
   * in the inner idle loop.
   *
   * This macro provides the way out:  RCU_NONIDLE(do_something_with_RCU())
   * will tell RCU that it needs to pay attending, invoke its argument
   * (in this example, a call to the do_something_with_RCU() function),
   * and then tell RCU to go back to ignoring this CPU.  It is permissible
   * to nest RCU_NONIDLE() wrappers, but the nesting level is currently
   * quite limited.  If deeper nesting is required, it will be necessary
   * to adjust DYNTICK_TASK_NESTING_VALUE accordingly.
   */
  #define RCU_NONIDLE(a) \
  	do { \
  		rcu_irq_enter(); \
  		do { a; } while (0); \
  		rcu_irq_exit(); \
  	} while (0)
  
  /*
   * Note a voluntary context switch for RCU-tasks benefit.  This is a
   * macro rather than an inline function to avoid #include hell.
   */
  #ifdef CONFIG_TASKS_RCU
  #define TASKS_RCU(x) x
  extern struct srcu_struct tasks_rcu_exit_srcu;
  #define rcu_note_voluntary_context_switch(t) \
  	do { \
  		rcu_all_qs(); \
  		if (ACCESS_ONCE((t)->rcu_tasks_holdout)) \
  			ACCESS_ONCE((t)->rcu_tasks_holdout) = false; \
  	} while (0)
  #else /* #ifdef CONFIG_TASKS_RCU */
  #define TASKS_RCU(x) do { } while (0)
  #define rcu_note_voluntary_context_switch(t)	rcu_all_qs()
  #endif /* #else #ifdef CONFIG_TASKS_RCU */
  
  /**
   * cond_resched_rcu_qs - Report potential quiescent states to RCU
   *
   * This macro resembles cond_resched(), except that it is defined to
   * report potential quiescent states to RCU-tasks even if the cond_resched()
   * machinery were to be shut off, as some advocate for PREEMPT kernels.
   */
  #define cond_resched_rcu_qs() \
  do { \
  	if (!cond_resched()) \
  		rcu_note_voluntary_context_switch(current); \
  } while (0)
  
  #if defined(CONFIG_DEBUG_LOCK_ALLOC) || defined(CONFIG_RCU_TRACE) || defined(CONFIG_SMP)
  bool __rcu_is_watching(void);
  #endif /* #if defined(CONFIG_DEBUG_LOCK_ALLOC) || defined(CONFIG_RCU_TRACE) || defined(CONFIG_SMP) */
  
  /*
   * Infrastructure to implement the synchronize_() primitives in
   * TREE_RCU and rcu_barrier_() primitives in TINY_RCU.
   */
  
  typedef void call_rcu_func_t(struct rcu_head *head,
  			     void (*func)(struct rcu_head *head));
  void wait_rcu_gp(call_rcu_func_t crf);
  
  #if defined(CONFIG_TREE_RCU) || defined(CONFIG_PREEMPT_RCU)
  #include <linux/rcutree.h>
  #elif defined(CONFIG_TINY_RCU)
  #include <linux/rcutiny.h>
  #else
  #error "Unknown RCU implementation specified to kernel configuration"
  #endif
  
  /*
   * init_rcu_head_on_stack()/destroy_rcu_head_on_stack() are needed for dynamic
   * initialization and destruction of rcu_head on the stack. rcu_head structures
   * allocated dynamically in the heap or defined statically don't need any
   * initialization.
   */
  #ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD
  void init_rcu_head(struct rcu_head *head);
  void destroy_rcu_head(struct rcu_head *head);
  void init_rcu_head_on_stack(struct rcu_head *head);
  void destroy_rcu_head_on_stack(struct rcu_head *head);
  #else /* !CONFIG_DEBUG_OBJECTS_RCU_HEAD */
  static inline void init_rcu_head(struct rcu_head *head)
  {
  }
  
  static inline void destroy_rcu_head(struct rcu_head *head)
  {
  }
  
  static inline void init_rcu_head_on_stack(struct rcu_head *head)
  {
  }
  
  static inline void destroy_rcu_head_on_stack(struct rcu_head *head)
  {
  }
  #endif	/* #else !CONFIG_DEBUG_OBJECTS_RCU_HEAD */
  
  #if defined(CONFIG_HOTPLUG_CPU) && defined(CONFIG_PROVE_RCU)
  bool rcu_lockdep_current_cpu_online(void);
  #else /* #if defined(CONFIG_HOTPLUG_CPU) && defined(CONFIG_PROVE_RCU) */
  static inline bool rcu_lockdep_current_cpu_online(void)
  {
  	return true;
  }
  #endif /* #else #if defined(CONFIG_HOTPLUG_CPU) && defined(CONFIG_PROVE_RCU) */
  
  #ifdef CONFIG_DEBUG_LOCK_ALLOC
  
  static inline void rcu_lock_acquire(struct lockdep_map *map)
  {
  	lock_acquire(map, 0, 0, 2, 0, NULL, _THIS_IP_);
  }
  
  static inline void rcu_lock_release(struct lockdep_map *map)
  {
  	lock_release(map, 1, _THIS_IP_);
  }
  
  extern struct lockdep_map rcu_lock_map;
  extern struct lockdep_map rcu_bh_lock_map;
  extern struct lockdep_map rcu_sched_lock_map;
  extern struct lockdep_map rcu_callback_map;
  int debug_lockdep_rcu_enabled(void);
  
  int rcu_read_lock_held(void);
  int rcu_read_lock_bh_held(void);
  
  /**
   * rcu_read_lock_sched_held() - might we be in RCU-sched read-side critical section?
   *
   * If CONFIG_DEBUG_LOCK_ALLOC is selected, returns nonzero iff in an
   * RCU-sched read-side critical section.  In absence of
   * CONFIG_DEBUG_LOCK_ALLOC, this assumes we are in an RCU-sched read-side
   * critical section unless it can prove otherwise.  Note that disabling
   * of preemption (including disabling irqs) counts as an RCU-sched
   * read-side critical section.  This is useful for debug checks in functions
   * that required that they be called within an RCU-sched read-side
   * critical section.
   *
   * Check debug_lockdep_rcu_enabled() to prevent false positives during boot
   * and while lockdep is disabled.
   *
   * Note that if the CPU is in the idle loop from an RCU point of
   * view (ie: that we are in the section between rcu_idle_enter() and
   * rcu_idle_exit()) then rcu_read_lock_held() returns false even if the CPU
   * did an rcu_read_lock().  The reason for this is that RCU ignores CPUs
   * that are in such a section, considering these as in extended quiescent
   * state, so such a CPU is effectively never in an RCU read-side critical
   * section regardless of what RCU primitives it invokes.  This state of
   * affairs is required --- we need to keep an RCU-free window in idle
   * where the CPU may possibly enter into low power mode. This way we can
   * notice an extended quiescent state to other CPUs that started a grace
   * period. Otherwise we would delay any grace period as long as we run in
   * the idle task.
   *
   * Similarly, we avoid claiming an SRCU read lock held if the current
   * CPU is offline.
   */
  #ifdef CONFIG_PREEMPT_COUNT
  static inline int rcu_read_lock_sched_held(void)
  {
  	int lockdep_opinion = 0;
  
  	if (!debug_lockdep_rcu_enabled())
  		return 1;
  	if (!rcu_is_watching())
  		return 0;
  	if (!rcu_lockdep_current_cpu_online())
  		return 0;
  	if (debug_locks)
  		lockdep_opinion = lock_is_held(&rcu_sched_lock_map);
  	return lockdep_opinion || preempt_count() != 0 || irqs_disabled();
  }
  #else /* #ifdef CONFIG_PREEMPT_COUNT */
  static inline int rcu_read_lock_sched_held(void)
  {
  	return 1;
  }
  #endif /* #else #ifdef CONFIG_PREEMPT_COUNT */
  
  #else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
  
  # define rcu_lock_acquire(a)		do { } while (0)
  # define rcu_lock_release(a)		do { } while (0)
  
  static inline int rcu_read_lock_held(void)
  {
  	return 1;
  }
  
  static inline int rcu_read_lock_bh_held(void)
  {
  	return 1;
  }
  
  #ifdef CONFIG_PREEMPT_COUNT
  static inline int rcu_read_lock_sched_held(void)
  {
  	return preempt_count() != 0 || irqs_disabled();
  }
  #else /* #ifdef CONFIG_PREEMPT_COUNT */
  static inline int rcu_read_lock_sched_held(void)
  {
  	return 1;
  }
  #endif /* #else #ifdef CONFIG_PREEMPT_COUNT */
  
  #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
  
  #ifdef CONFIG_PROVE_RCU
  
  /**
   * rcu_lockdep_assert - emit lockdep splat if specified condition not met
   * @c: condition to check
   * @s: informative message
   */
  #define rcu_lockdep_assert(c, s)					\
  	do {								\
  		static bool __section(.data.unlikely) __warned;		\
  		if (debug_lockdep_rcu_enabled() && !__warned && !(c)) {	\
  			__warned = true;				\
  			lockdep_rcu_suspicious(__FILE__, __LINE__, s);	\
  		}							\
  	} while (0)
  
  #if defined(CONFIG_PROVE_RCU) && !defined(CONFIG_PREEMPT_RCU)
  static inline void rcu_preempt_sleep_check(void)
  {
  	rcu_lockdep_assert(!lock_is_held(&rcu_lock_map),
  			   "Illegal context switch in RCU read-side critical section");
  }
  #else /* #ifdef CONFIG_PROVE_RCU */
  static inline void rcu_preempt_sleep_check(void)
  {
  }
  #endif /* #else #ifdef CONFIG_PROVE_RCU */
  
  #define rcu_sleep_check()						\
  	do {								\
  		rcu_preempt_sleep_check();				\
  		rcu_lockdep_assert(!lock_is_held(&rcu_bh_lock_map),	\
  				   "Illegal context switch in RCU-bh read-side critical section"); \
  		rcu_lockdep_assert(!lock_is_held(&rcu_sched_lock_map),	\
  				   "Illegal context switch in RCU-sched read-side critical section"); \
  	} while (0)
  
  #else /* #ifdef CONFIG_PROVE_RCU */
  
  #define rcu_lockdep_assert(c, s) do { } while (0)
  #define rcu_sleep_check() do { } while (0)
  
  #endif /* #else #ifdef CONFIG_PROVE_RCU */
  
  /*
   * Helper functions for rcu_dereference_check(), rcu_dereference_protected()
   * and rcu_assign_pointer().  Some of these could be folded into their
   * callers, but they are left separate in order to ease introduction of
   * multiple flavors of pointers to match the multiple flavors of RCU
   * (e.g., __rcu_bh, * __rcu_sched, and __srcu), should this make sense in
   * the future.
   */
  
  #ifdef __CHECKER__
  #define rcu_dereference_sparse(p, space) \
  	((void)(((typeof(*p) space *)p) == p))
  #else /* #ifdef __CHECKER__ */
  #define rcu_dereference_sparse(p, space)
  #endif /* #else #ifdef __CHECKER__ */
  
  #define __rcu_access_pointer(p, space) \
  ({ \
  	typeof(*p) *_________p1 = (typeof(*p) *__force)ACCESS_ONCE(p); \
  	rcu_dereference_sparse(p, space); \
  	((typeof(*p) __force __kernel *)(_________p1)); \
  })
  #define __rcu_dereference_check(p, c, space) \
  ({ \
  	/* Dependency order vs. p above. */ \
  	typeof(*p) *________p1 = (typeof(*p) *__force)lockless_dereference(p); \
  	rcu_lockdep_assert(c, "suspicious rcu_dereference_check() usage"); \
  	rcu_dereference_sparse(p, space); \
  	((typeof(*p) __force __kernel *)(________p1)); \
  })
  #define __rcu_dereference_protected(p, c, space) \
  ({ \
  	rcu_lockdep_assert(c, "suspicious rcu_dereference_protected() usage"); \
  	rcu_dereference_sparse(p, space); \
  	((typeof(*p) __force __kernel *)(p)); \
  })
  
  #define __rcu_access_index(p, space) \
  ({ \
  	typeof(p) _________p1 = ACCESS_ONCE(p); \
  	rcu_dereference_sparse(p, space); \
  	(_________p1); \
  })
  #define __rcu_dereference_index_check(p, c) \
  ({ \
  	/* Dependency order vs. p above. */ \
  	typeof(p) _________p1 = lockless_dereference(p); \
  	rcu_lockdep_assert(c, \
  			   "suspicious rcu_dereference_index_check() usage"); \
  	(_________p1); \
  })
  
  /**
   * RCU_INITIALIZER() - statically initialize an RCU-protected global variable
   * @v: The value to statically initialize with.
   */
  #define RCU_INITIALIZER(v) (typeof(*(v)) __force __rcu *)(v)
  
  /**
   * lockless_dereference() - safely load a pointer for later dereference
   * @p: The pointer to load
   *
   * Similar to rcu_dereference(), but for situations where the pointed-to
   * object's lifetime is managed by something other than RCU.  That
   * "something other" might be reference counting or simple immortality.
   */
  #define lockless_dereference(p) \
  ({ \
  	typeof(p) _________p1 = ACCESS_ONCE(p); \
  	smp_read_barrier_depends(); /* Dependency order vs. p above. */ \
  	(_________p1); \
  })
  
  /**
   * rcu_assign_pointer() - assign to RCU-protected pointer
   * @p: pointer to assign to
   * @v: value to assign (publish)
   *
   * Assigns the specified value to the specified RCU-protected
   * pointer, ensuring that any concurrent RCU readers will see
   * any prior initialization.
   *
   * Inserts memory barriers on architectures that require them
   * (which is most of them), and also prevents the compiler from
   * reordering the code that initializes the structure after the pointer
   * assignment.  More importantly, this call documents which pointers
   * will be dereferenced by RCU read-side code.
   *
   * In some special cases, you may use RCU_INIT_POINTER() instead
   * of rcu_assign_pointer().  RCU_INIT_POINTER() is a bit faster due
   * to the fact that it does not constrain either the CPU or the compiler.
   * That said, using RCU_INIT_POINTER() when you should have used
   * rcu_assign_pointer() is a very bad thing that results in
   * impossible-to-diagnose memory corruption.  So please be careful.
   * See the RCU_INIT_POINTER() comment header for details.
   *
   * Note that rcu_assign_pointer() evaluates each of its arguments only
   * once, appearances notwithstanding.  One of the "extra" evaluations
   * is in typeof() and the other visible only to sparse (__CHECKER__),
   * neither of which actually execute the argument.  As with most cpp
   * macros, this execute-arguments-only-once property is important, so
   * please be careful when making changes to rcu_assign_pointer() and the
   * other macros that it invokes.
   */
  #define rcu_assign_pointer(p, v) smp_store_release(&p, RCU_INITIALIZER(v))
  
  /**
   * rcu_access_pointer() - fetch RCU pointer with no dereferencing
   * @p: The pointer to read
   *
   * Return the value of the specified RCU-protected pointer, but omit the
   * smp_read_barrier_depends() and keep the ACCESS_ONCE().  This is useful
   * when the value of this pointer is accessed, but the pointer is not
   * dereferenced, for example, when testing an RCU-protected pointer against
   * NULL.  Although rcu_access_pointer() may also be used in cases where
   * update-side locks prevent the value of the pointer from changing, you
   * should instead use rcu_dereference_protected() for this use case.
   *
   * It is also permissible to use rcu_access_pointer() when read-side
   * access to the pointer was removed at least one grace period ago, as
   * is the case in the context of the RCU callback that is freeing up
   * the data, or after a synchronize_rcu() returns.  This can be useful
   * when tearing down multi-linked structures after a grace period
   * has elapsed.
   */
  #define rcu_access_pointer(p) __rcu_access_pointer((p), __rcu)
  
  /**
   * rcu_dereference_check() - rcu_dereference with debug checking
   * @p: The pointer to read, prior to dereferencing
   * @c: The conditions under which the dereference will take place
   *
   * Do an rcu_dereference(), but check that the conditions under which the
   * dereference will take place are correct.  Typically the conditions
   * indicate the various locking conditions that should be held at that
   * point.  The check should return true if the conditions are satisfied.
   * An implicit check for being in an RCU read-side critical section
   * (rcu_read_lock()) is included.
   *
   * For example:
   *
   *	bar = rcu_dereference_check(foo->bar, lockdep_is_held(&foo->lock));
   *
   * could be used to indicate to lockdep that foo->bar may only be dereferenced
   * if either rcu_read_lock() is held, or that the lock required to replace
   * the bar struct at foo->bar is held.
   *
   * Note that the list of conditions may also include indications of when a lock
   * need not be held, for example during initialisation or destruction of the
   * target struct:
   *
   *	bar = rcu_dereference_check(foo->bar, lockdep_is_held(&foo->lock) ||
   *					      atomic_read(&foo->usage) == 0);
   *
   * Inserts memory barriers on architectures that require them
   * (currently only the Alpha), prevents the compiler from refetching
   * (and from merging fetches), and, more importantly, documents exactly
   * which pointers are protected by RCU and checks that the pointer is
   * annotated as __rcu.
   */
  #define rcu_dereference_check(p, c) \
  	__rcu_dereference_check((p), rcu_read_lock_held() || (c), __rcu)
  
  /**
   * rcu_dereference_bh_check() - rcu_dereference_bh with debug checking
   * @p: The pointer to read, prior to dereferencing
   * @c: The conditions under which the dereference will take place
   *
   * This is the RCU-bh counterpart to rcu_dereference_check().
   */
  #define rcu_dereference_bh_check(p, c) \
  	__rcu_dereference_check((p), rcu_read_lock_bh_held() || (c), __rcu)
  
  /**
   * rcu_dereference_sched_check() - rcu_dereference_sched with debug checking
   * @p: The pointer to read, prior to dereferencing
   * @c: The conditions under which the dereference will take place
   *
   * This is the RCU-sched counterpart to rcu_dereference_check().
   */
  #define rcu_dereference_sched_check(p, c) \
  	__rcu_dereference_check((p), rcu_read_lock_sched_held() || (c), \
  				__rcu)
  
  #define rcu_dereference_raw(p) rcu_dereference_check(p, 1) /*@@@ needed? @@@*/
  
  /*
   * The tracing infrastructure traces RCU (we want that), but unfortunately
   * some of the RCU checks causes tracing to lock up the system.
   *
   * The tracing version of rcu_dereference_raw() must not call
   * rcu_read_lock_held().
   */
  #define rcu_dereference_raw_notrace(p) __rcu_dereference_check((p), 1, __rcu)
  
  /**
   * rcu_access_index() - fetch RCU index with no dereferencing
   * @p: The index to read
   *
   * Return the value of the specified RCU-protected index, but omit the
   * smp_read_barrier_depends() and keep the ACCESS_ONCE().  This is useful
   * when the value of this index is accessed, but the index is not
   * dereferenced, for example, when testing an RCU-protected index against
   * -1.  Although rcu_access_index() may also be used in cases where
   * update-side locks prevent the value of the index from changing, you
   * should instead use rcu_dereference_index_protected() for this use case.
   */
  #define rcu_access_index(p) __rcu_access_index((p), __rcu)
  
  /**
   * rcu_dereference_index_check() - rcu_dereference for indices with debug checking
   * @p: The pointer to read, prior to dereferencing
   * @c: The conditions under which the dereference will take place
   *
   * Similar to rcu_dereference_check(), but omits the sparse checking.
   * This allows rcu_dereference_index_check() to be used on integers,
   * which can then be used as array indices.  Attempting to use
   * rcu_dereference_check() on an integer will give compiler warnings
   * because the sparse address-space mechanism relies on dereferencing
   * the RCU-protected pointer.  Dereferencing integers is not something
   * that even gcc will put up with.
   *
   * Note that this function does not implicitly check for RCU read-side
   * critical sections.  If this function gains lots of uses, it might
   * make sense to provide versions for each flavor of RCU, but it does
   * not make sense as of early 2010.
   */
  #define rcu_dereference_index_check(p, c) \
  	__rcu_dereference_index_check((p), (c))
  
  /**
   * rcu_dereference_protected() - fetch RCU pointer when updates prevented
   * @p: The pointer to read, prior to dereferencing
   * @c: The conditions under which the dereference will take place
   *
   * Return the value of the specified RCU-protected pointer, but omit
   * both the smp_read_barrier_depends() and the ACCESS_ONCE().  This
   * is useful in cases where update-side locks prevent the value of the
   * pointer from changing.  Please note that this primitive does -not-
   * prevent the compiler from repeating this reference or combining it
   * with other references, so it should not be used without protection
   * of appropriate locks.
   *
   * This function is only for update-side use.  Using this function
   * when protected only by rcu_read_lock() will result in infrequent
   * but very ugly failures.
   */
  #define rcu_dereference_protected(p, c) \
  	__rcu_dereference_protected((p), (c), __rcu)
  
  
  /**
   * rcu_dereference() - fetch RCU-protected pointer for dereferencing
   * @p: The pointer to read, prior to dereferencing
   *
   * This is a simple wrapper around rcu_dereference_check().
   */
  #define rcu_dereference(p) rcu_dereference_check(p, 0)
  
  /**
   * rcu_dereference_bh() - fetch an RCU-bh-protected pointer for dereferencing
   * @p: The pointer to read, prior to dereferencing
   *
   * Makes rcu_dereference_check() do the dirty work.
   */
  #define rcu_dereference_bh(p) rcu_dereference_bh_check(p, 0)
  
  /**
   * rcu_dereference_sched() - fetch RCU-sched-protected pointer for dereferencing
   * @p: The pointer to read, prior to dereferencing
   *
   * Makes rcu_dereference_check() do the dirty work.
   */
  #define rcu_dereference_sched(p) rcu_dereference_sched_check(p, 0)
  
  /**
   * rcu_read_lock() - mark the beginning of an RCU read-side critical section
   *
   * When synchronize_rcu() is invoked on one CPU while other CPUs
   * are within RCU read-side critical sections, then the
   * synchronize_rcu() is guaranteed to block until after all the other
   * CPUs exit their critical sections.  Similarly, if call_rcu() is invoked
   * on one CPU while other CPUs are within RCU read-side critical
   * sections, invocation of the corresponding RCU callback is deferred
   * until after the all the other CPUs exit their critical sections.
   *
   * Note, however, that RCU callbacks are permitted to run concurrently
   * with new RCU read-side critical sections.  One way that this can happen
   * is via the following sequence of events: (1) CPU 0 enters an RCU
   * read-side critical section, (2) CPU 1 invokes call_rcu() to register
   * an RCU callback, (3) CPU 0 exits the RCU read-side critical section,
   * (4) CPU 2 enters a RCU read-side critical section, (5) the RCU
   * callback is invoked.  This is legal, because the RCU read-side critical
   * section that was running concurrently with the call_rcu() (and which
   * therefore might be referencing something that the corresponding RCU
   * callback would free up) has completed before the corresponding
   * RCU callback is invoked.
   *
   * RCU read-side critical sections may be nested.  Any deferred actions
   * will be deferred until the outermost RCU read-side critical section
   * completes.
   *
   * You can avoid reading and understanding the next paragraph by
   * following this rule: don't put anything in an rcu_read_lock() RCU
   * read-side critical section that would block in a !PREEMPT kernel.
   * But if you want the full story, read on!
   *
   * In non-preemptible RCU implementations (TREE_RCU and TINY_RCU),
   * it is illegal to block while in an RCU read-side critical section.
   * In preemptible RCU implementations (PREEMPT_RCU) in CONFIG_PREEMPT
   * kernel builds, RCU read-side critical sections may be preempted,
   * but explicit blocking is illegal.  Finally, in preemptible RCU
   * implementations in real-time (with -rt patchset) kernel builds, RCU
   * read-side critical sections may be preempted and they may also block, but
   * only when acquiring spinlocks that are subject to priority inheritance.
   */
  static inline void rcu_read_lock(void)
  {
  	__rcu_read_lock();
  	__acquire(RCU);
  	rcu_lock_acquire(&rcu_lock_map);
  	rcu_lockdep_assert(rcu_is_watching(),
  			   "rcu_read_lock() used illegally while idle");
  }
  
  /*
   * So where is rcu_write_lock()?  It does not exist, as there is no
   * way for writers to lock out RCU readers.  This is a feature, not
   * a bug -- this property is what provides RCU's performance benefits.
   * Of course, writers must coordinate with each other.  The normal
   * spinlock primitives work well for this, but any other technique may be
   * used as well.  RCU does not care how the writers keep out of each
   * others' way, as long as they do so.
   */
  
  /**
   * rcu_read_unlock() - marks the end of an RCU read-side critical section.
   *
   * In most situations, rcu_read_unlock() is immune from deadlock.
   * However, in kernels built with CONFIG_RCU_BOOST, rcu_read_unlock()
   * is responsible for deboosting, which it does via rt_mutex_unlock().
   * Unfortunately, this function acquires the scheduler's runqueue and
   * priority-inheritance spinlocks.  This means that deadlock could result
   * if the caller of rcu_read_unlock() already holds one of these locks or
   * any lock that is ever acquired while holding them; or any lock which
   * can be taken from interrupt context because rcu_boost()->rt_mutex_lock()
   * does not disable irqs while taking ->wait_lock.
   *
   * That said, RCU readers are never priority boosted unless they were
   * preempted.  Therefore, one way to avoid deadlock is to make sure
   * that preemption never happens within any RCU read-side critical
   * section whose outermost rcu_read_unlock() is called with one of
   * rt_mutex_unlock()'s locks held.  Such preemption can be avoided in
   * a number of ways, for example, by invoking preempt_disable() before
   * critical section's outermost rcu_read_lock().
   *
   * Given that the set of locks acquired by rt_mutex_unlock() might change
   * at any time, a somewhat more future-proofed approach is to make sure
   * that that preemption never happens within any RCU read-side critical
   * section whose outermost rcu_read_unlock() is called with irqs disabled.
   * This approach relies on the fact that rt_mutex_unlock() currently only
   * acquires irq-disabled locks.
   *
   * The second of these two approaches is best in most situations,
   * however, the first approach can also be useful, at least to those
   * developers willing to keep abreast of the set of locks acquired by
   * rt_mutex_unlock().
   *
   * See rcu_read_lock() for more information.
   */
  static inline void rcu_read_unlock(void)
  {
  	rcu_lockdep_assert(rcu_is_watching(),
  			   "rcu_read_unlock() used illegally while idle");
  	rcu_lock_release(&rcu_lock_map);
  	__release(RCU);
  	__rcu_read_unlock();
  }
  
  /**
   * rcu_read_lock_bh() - mark the beginning of an RCU-bh critical section
   *
   * This is equivalent of rcu_read_lock(), but to be used when updates
   * are being done using call_rcu_bh() or synchronize_rcu_bh(). Since
   * both call_rcu_bh() and synchronize_rcu_bh() consider completion of a
   * softirq handler to be a quiescent state, a process in RCU read-side
   * critical section must be protected by disabling softirqs. Read-side
   * critical sections in interrupt context can use just rcu_read_lock(),
   * though this should at least be commented to avoid confusing people
   * reading the code.
   *
   * Note that rcu_read_lock_bh() and the matching rcu_read_unlock_bh()
   * must occur in the same context, for example, it is illegal to invoke
   * rcu_read_unlock_bh() from one task if the matching rcu_read_lock_bh()
   * was invoked from some other task.
   */
  static inline void rcu_read_lock_bh(void)
  {
  	local_bh_disable();
  	__acquire(RCU_BH);
  	rcu_lock_acquire(&rcu_bh_lock_map);
  	rcu_lockdep_assert(rcu_is_watching(),
  			   "rcu_read_lock_bh() used illegally while idle");
  }
  
  /*
   * rcu_read_unlock_bh - marks the end of a softirq-only RCU critical section
   *
   * See rcu_read_lock_bh() for more information.
   */
  static inline void rcu_read_unlock_bh(void)
  {
  	rcu_lockdep_assert(rcu_is_watching(),
  			   "rcu_read_unlock_bh() used illegally while idle");
  	rcu_lock_release(&rcu_bh_lock_map);
  	__release(RCU_BH);
  	local_bh_enable();
  }
  
  /**
   * rcu_read_lock_sched() - mark the beginning of a RCU-sched critical section
   *
   * This is equivalent of rcu_read_lock(), but to be used when updates
   * are being done using call_rcu_sched() or synchronize_rcu_sched().
   * Read-side critical sections can also be introduced by anything that
   * disables preemption, including local_irq_disable() and friends.
   *
   * Note that rcu_read_lock_sched() and the matching rcu_read_unlock_sched()
   * must occur in the same context, for example, it is illegal to invoke
   * rcu_read_unlock_sched() from process context if the matching
   * rcu_read_lock_sched() was invoked from an NMI handler.
   */
  static inline void rcu_read_lock_sched(void)
  {
  	preempt_disable();
  	__acquire(RCU_SCHED);
  	rcu_lock_acquire(&rcu_sched_lock_map);
  	rcu_lockdep_assert(rcu_is_watching(),
  			   "rcu_read_lock_sched() used illegally while idle");
  }
  
  /* Used by lockdep and tracing: cannot be traced, cannot call lockdep. */
  static inline notrace void rcu_read_lock_sched_notrace(void)
  {
  	preempt_disable_notrace();
  	__acquire(RCU_SCHED);
  }
  
  /*
   * rcu_read_unlock_sched - marks the end of a RCU-classic critical section
   *
   * See rcu_read_lock_sched for more information.
   */
  static inline void rcu_read_unlock_sched(void)
  {
  	rcu_lockdep_assert(rcu_is_watching(),
  			   "rcu_read_unlock_sched() used illegally while idle");
  	rcu_lock_release(&rcu_sched_lock_map);
  	__release(RCU_SCHED);
  	preempt_enable();
  }
  
  /* Used by lockdep and tracing: cannot be traced, cannot call lockdep. */
  static inline notrace void rcu_read_unlock_sched_notrace(void)
  {
  	__release(RCU_SCHED);
  	preempt_enable_notrace();
  }
  
  /**
   * RCU_INIT_POINTER() - initialize an RCU protected pointer
   *
   * Initialize an RCU-protected pointer in special cases where readers
   * do not need ordering constraints on the CPU or the compiler.  These
   * special cases are:
   *
   * 1.	This use of RCU_INIT_POINTER() is NULLing out the pointer -or-
   * 2.	The caller has taken whatever steps are required to prevent
   *	RCU readers from concurrently accessing this pointer -or-
   * 3.	The referenced data structure has already been exposed to
   *	readers either at compile time or via rcu_assign_pointer() -and-
   *	a.	You have not made -any- reader-visible changes to
   *		this structure since then -or-
   *	b.	It is OK for readers accessing this structure from its
   *		new location to see the old state of the structure.  (For
   *		example, the changes were to statistical counters or to
   *		other state where exact synchronization is not required.)
   *
   * Failure to follow these rules governing use of RCU_INIT_POINTER() will
   * result in impossible-to-diagnose memory corruption.  As in the structures
   * will look OK in crash dumps, but any concurrent RCU readers might
   * see pre-initialized values of the referenced data structure.  So
   * please be very careful how you use RCU_INIT_POINTER()!!!
   *
   * If you are creating an RCU-protected linked structure that is accessed
   * by a single external-to-structure RCU-protected pointer, then you may
   * use RCU_INIT_POINTER() to initialize the internal RCU-protected
   * pointers, but you must use rcu_assign_pointer() to initialize the
   * external-to-structure pointer -after- you have completely initialized
   * the reader-accessible portions of the linked structure.
   *
   * Note that unlike rcu_assign_pointer(), RCU_INIT_POINTER() provides no
   * ordering guarantees for either the CPU or the compiler.
   */
  #define RCU_INIT_POINTER(p, v) \
  	do { \
  		rcu_dereference_sparse(p, __rcu); \
  		p = RCU_INITIALIZER(v); \
  	} while (0)
  
  /**
   * RCU_POINTER_INITIALIZER() - statically initialize an RCU protected pointer
   *
   * GCC-style initialization for an RCU-protected pointer in a structure field.
   */
  #define RCU_POINTER_INITIALIZER(p, v) \
  		.p = RCU_INITIALIZER(v)
  
  /*
   * Does the specified offset indicate that the corresponding rcu_head
   * structure can be handled by kfree_rcu()?
   */
  #define __is_kfree_rcu_offset(offset) ((offset) < 4096)
  
  /*
   * Helper macro for kfree_rcu() to prevent argument-expansion eyestrain.
   */
  #define __kfree_rcu(head, offset) \
  	do { \
  		BUILD_BUG_ON(!__is_kfree_rcu_offset(offset)); \
  		kfree_call_rcu(head, (void (*)(struct rcu_head *))(unsigned long)(offset)); \
  	} while (0)
  
  /**
   * kfree_rcu() - kfree an object after a grace period.
   * @ptr:	pointer to kfree
   * @rcu_head:	the name of the struct rcu_head within the type of @ptr.
   *
   * Many rcu callbacks functions just call kfree() on the base structure.
   * These functions are trivial, but their size adds up, and furthermore
   * when they are used in a kernel module, that module must invoke the
   * high-latency rcu_barrier() function at module-unload time.
   *
   * The kfree_rcu() function handles this issue.  Rather than encoding a
   * function address in the embedded rcu_head structure, kfree_rcu() instead
   * encodes the offset of the rcu_head structure within the base structure.
   * Because the functions are not allowed in the low-order 4096 bytes of
   * kernel virtual memory, offsets up to 4095 bytes can be accommodated.
   * If the offset is larger than 4095 bytes, a compile-time error will
   * be generated in __kfree_rcu().  If this error is triggered, you can
   * either fall back to use of call_rcu() or rearrange the structure to
   * position the rcu_head structure into the first 4096 bytes.
   *
   * Note that the allowable offset might decrease in the future, for example,
   * to allow something like kmem_cache_free_rcu().
   *
   * The BUILD_BUG_ON check must not involve any function calls, hence the
   * checks are done in macros here.
   */
  #define kfree_rcu(ptr, rcu_head)					\
  	__kfree_rcu(&((ptr)->rcu_head), offsetof(typeof(*(ptr)), rcu_head))
  
  #if defined(CONFIG_TINY_RCU) || defined(CONFIG_RCU_NOCB_CPU_ALL)
  static inline int rcu_needs_cpu(unsigned long *delta_jiffies)
  {
  	*delta_jiffies = ULONG_MAX;
  	return 0;
  }
  #endif /* #if defined(CONFIG_TINY_RCU) || defined(CONFIG_RCU_NOCB_CPU_ALL) */
  
  #if defined(CONFIG_RCU_NOCB_CPU_ALL)
  static inline bool rcu_is_nocb_cpu(int cpu) { return true; }
  #elif defined(CONFIG_RCU_NOCB_CPU)
  bool rcu_is_nocb_cpu(int cpu);
  #else
  static inline bool rcu_is_nocb_cpu(int cpu) { return false; }
  #endif
  
  
  /* Only for use by adaptive-ticks code. */
  #ifdef CONFIG_NO_HZ_FULL_SYSIDLE
  bool rcu_sys_is_idle(void);
  void rcu_sysidle_force_exit(void);
  #else /* #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
  
  static inline bool rcu_sys_is_idle(void)
  {
  	return false;
  }
  
  static inline void rcu_sysidle_force_exit(void)
  {
  }
  
  #endif /* #else #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
  
  
  #endif /* __LINUX_RCUPDATE_H */
  