/*
   * Sleepable 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, write to the Free Software
   * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
   *
   * Copyright (C) IBM Corporation, 2006

   * Copyright (C) Fujitsu, 2012

   *
   * Author: Paul McKenney <paulmck@us.ibm.com>

   *	   Lai Jiangshan <laijs@cn.fujitsu.com>

   *
   * For detailed explanation of Read-Copy Update mechanism see -
   * 		Documentation/RCU/ *.txt
   *
   */

  #include <linux/export.h>

  #include <linux/mutex.h>
  #include <linux/percpu.h>
  #include <linux/preempt.h>
  #include <linux/rcupdate.h>
  #include <linux/sched.h>

  #include <linux/smp.h>

  #include <linux/delay.h>

  #include <linux/srcu.h>

  #include <trace/events/rcu.h>
  
  #include "rcu.h"

  /*
   * Initialize an rcu_batch structure to empty.
   */
  static inline void rcu_batch_init(struct rcu_batch *b)
  {
  	b->head = NULL;
  	b->tail = &b->head;
  }
  
  /*
   * Enqueue a callback onto the tail of the specified rcu_batch structure.
   */
  static inline void rcu_batch_queue(struct rcu_batch *b, struct rcu_head *head)
  {
  	*b->tail = head;
  	b->tail = &head->next;
  }
  
  /*
   * Is the specified rcu_batch structure empty?
   */
  static inline bool rcu_batch_empty(struct rcu_batch *b)
  {
  	return b->tail == &b->head;
  }
  
  /*
   * Remove the callback at the head of the specified rcu_batch structure
   * and return a pointer to it, or return NULL if the structure is empty.
   */
  static inline struct rcu_head *rcu_batch_dequeue(struct rcu_batch *b)
  {
  	struct rcu_head *head;
  
  	if (rcu_batch_empty(b))
  		return NULL;
  
  	head = b->head;
  	b->head = head->next;
  	if (b->tail == &head->next)
  		rcu_batch_init(b);
  
  	return head;
  }
  
  /*
   * Move all callbacks from the rcu_batch structure specified by "from" to
   * the structure specified by "to".
   */
  static inline void rcu_batch_move(struct rcu_batch *to, struct rcu_batch *from)
  {
  	if (!rcu_batch_empty(from)) {
  		*to->tail = from->head;
  		to->tail = from->tail;
  		rcu_batch_init(from);
  	}
  }

  static int init_srcu_struct_fields(struct srcu_struct *sp)
  {
  	sp->completed = 0;

  	spin_lock_init(&sp->queue_lock);
  	sp->running = false;
  	rcu_batch_init(&sp->batch_queue);
  	rcu_batch_init(&sp->batch_check0);
  	rcu_batch_init(&sp->batch_check1);
  	rcu_batch_init(&sp->batch_done);
  	INIT_DELAYED_WORK(&sp->work, process_srcu);

  	sp->per_cpu_ref = alloc_percpu(struct srcu_struct_array);
  	return sp->per_cpu_ref ? 0 : -ENOMEM;
  }
  
  #ifdef CONFIG_DEBUG_LOCK_ALLOC
  
  int __init_srcu_struct(struct srcu_struct *sp, const char *name,
  		       struct lock_class_key *key)
  {

  	/* Don't re-initialize a lock while it is held. */
  	debug_check_no_locks_freed((void *)sp, sizeof(*sp));
  	lockdep_init_map(&sp->dep_map, name, key, 0);

  	return init_srcu_struct_fields(sp);
  }
  EXPORT_SYMBOL_GPL(__init_srcu_struct);
  
  #else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */

  /**
   * init_srcu_struct - initialize a sleep-RCU structure
   * @sp: structure to initialize.
   *
   * Must invoke this on a given srcu_struct before passing that srcu_struct
   * to any other function.  Each srcu_struct represents a separate domain
   * of SRCU protection.
   */

  int init_srcu_struct(struct srcu_struct *sp)

  {

  	return init_srcu_struct_fields(sp);

  }

  EXPORT_SYMBOL_GPL(init_srcu_struct);

  #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */

  /*

   * Returns approximate total of the readers' ->seq[] values for the
   * rank of per-CPU counters specified by idx.
   */
  static unsigned long srcu_readers_seq_idx(struct srcu_struct *sp, int idx)
  {
  	int cpu;
  	unsigned long sum = 0;
  	unsigned long t;
  
  	for_each_possible_cpu(cpu) {
  		t = ACCESS_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->seq[idx]);
  		sum += t;
  	}
  	return sum;
  }
  
  /*

   * Returns approximate number of readers active on the specified rank

   * of the per-CPU ->c[] counters.

   */

  static unsigned long srcu_readers_active_idx(struct srcu_struct *sp, int idx)
  {
  	int cpu;
  	unsigned long sum = 0;
  	unsigned long t;

  	for_each_possible_cpu(cpu) {
  		t = ACCESS_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[idx]);
  		sum += t;

  	}

  	return sum;

  }
  
  /*

   * Return true if the number of pre-existing readers is determined to
   * be stably zero.  An example unstable zero can occur if the call
   * to srcu_readers_active_idx() misses an __srcu_read_lock() increment,
   * but due to task migration, sees the corresponding __srcu_read_unlock()
   * decrement.  This can happen because srcu_readers_active_idx() takes
   * time to sum the array, and might in fact be interrupted or preempted
   * partway through the summation.

   */
  static bool srcu_readers_active_idx_check(struct srcu_struct *sp, int idx)

  {

  	unsigned long seq;
  
  	seq = srcu_readers_seq_idx(sp, idx);
  
  	/*
  	 * The following smp_mb() A pairs with the smp_mb() B located in
  	 * __srcu_read_lock().  This pairing ensures that if an
  	 * __srcu_read_lock() increments its counter after the summation
  	 * in srcu_readers_active_idx(), then the corresponding SRCU read-side
  	 * critical section will see any changes made prior to the start
  	 * of the current SRCU grace period.
  	 *
  	 * Also, if the above call to srcu_readers_seq_idx() saw the
  	 * increment of ->seq[], then the call to srcu_readers_active_idx()
  	 * must see the increment of ->c[].
  	 */
  	smp_mb(); /* A */

  	/*
  	 * Note that srcu_readers_active_idx() can incorrectly return
  	 * zero even though there is a pre-existing reader throughout.
  	 * To see this, suppose that task A is in a very long SRCU
  	 * read-side critical section that started on CPU 0, and that

  	 * no other reader exists, so that the sum of the counters

  	 * is equal to one.  Then suppose that task B starts executing
  	 * srcu_readers_active_idx(), summing up to CPU 1, and then that
  	 * task C starts reading on CPU 0, so that its increment is not
  	 * summed, but finishes reading on CPU 2, so that its decrement
  	 * -is- summed.  Then when task B completes its sum, it will
  	 * incorrectly get zero, despite the fact that task A has been
  	 * in its SRCU read-side critical section the whole time.
  	 *
  	 * We therefore do a validation step should srcu_readers_active_idx()
  	 * return zero.
  	 */
  	if (srcu_readers_active_idx(sp, idx) != 0)
  		return false;
  
  	/*

  	 * The remainder of this function is the validation step.
  	 * The following smp_mb() D pairs with the smp_mb() C in
  	 * __srcu_read_unlock().  If the __srcu_read_unlock() was seen
  	 * by srcu_readers_active_idx() above, then any destructive
  	 * operation performed after the grace period will happen after
  	 * the corresponding SRCU read-side critical section.

  	 *

  	 * Note that there can be at most NR_CPUS worth of readers using
  	 * the old index, which is not enough to overflow even a 32-bit
  	 * integer.  (Yes, this does mean that systems having more than
  	 * a billion or so CPUs need to be 64-bit systems.)  Therefore,
  	 * the sum of the ->seq[] counters cannot possibly overflow.
  	 * Therefore, the only way that the return values of the two
  	 * calls to srcu_readers_seq_idx() can be equal is if there were
  	 * no increments of the corresponding rank of ->seq[] counts
  	 * in the interim.  But the missed-increment scenario laid out
  	 * above includes an increment of the ->seq[] counter by
  	 * the corresponding __srcu_read_lock().  Therefore, if this
  	 * scenario occurs, the return values from the two calls to
  	 * srcu_readers_seq_idx() will differ, and thus the validation
  	 * step below suffices.

  	 */

  	smp_mb(); /* D */
  
  	return srcu_readers_seq_idx(sp, idx) == seq;

  }
  
  /**
   * srcu_readers_active - returns approximate number of readers.
   * @sp: which srcu_struct to count active readers (holding srcu_read_lock).
   *
   * Note that this is not an atomic primitive, and can therefore suffer
   * severe errors when invoked on an active srcu_struct.  That said, it
   * can be useful as an error check at cleanup time.
   */

  static int srcu_readers_active(struct srcu_struct *sp)

  {

  	int cpu;
  	unsigned long sum = 0;
  
  	for_each_possible_cpu(cpu) {
  		sum += ACCESS_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[0]);
  		sum += ACCESS_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[1]);
  	}
  	return sum;

  }
  
  /**
   * cleanup_srcu_struct - deconstruct a sleep-RCU structure
   * @sp: structure to clean up.
   *
   * Must invoke this after you are finished using a given srcu_struct that
   * was initialized via init_srcu_struct(), else you leak memory.
   */
  void cleanup_srcu_struct(struct srcu_struct *sp)
  {

  	if (WARN_ON(srcu_readers_active(sp)))
  		return; /* Leakage unless caller handles error. */

  	free_percpu(sp->per_cpu_ref);
  	sp->per_cpu_ref = NULL;
  }

  EXPORT_SYMBOL_GPL(cleanup_srcu_struct);

  /*

   * Counts the new reader in the appropriate per-CPU element of the
   * srcu_struct.  Must be called from process context.
   * Returns an index that must be passed to the matching srcu_read_unlock().
   */

  int __srcu_read_lock(struct srcu_struct *sp)

  {
  	int idx;

  	idx = ACCESS_ONCE(sp->completed) & 0x1;

  	preempt_disable();

  	ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->c[idx]) += 1;

  	smp_mb(); /* B */  /* Avoid leaking the critical section. */

  	ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->seq[idx]) += 1;

  	preempt_enable();
  	return idx;
  }

  EXPORT_SYMBOL_GPL(__srcu_read_lock);

  /*

   * Removes the count for the old reader from the appropriate per-CPU
   * element of the srcu_struct.  Note that this may well be a different
   * CPU than that which was incremented by the corresponding srcu_read_lock().
   * Must be called from process context.
   */

  void __srcu_read_unlock(struct srcu_struct *sp, int idx)

  {

  	smp_mb(); /* C */  /* Avoid leaking the critical section. */

  	this_cpu_dec(sp->per_cpu_ref->c[idx]);

  }

  EXPORT_SYMBOL_GPL(__srcu_read_unlock);

  /*

   * We use an adaptive strategy for synchronize_srcu() and especially for
   * synchronize_srcu_expedited().  We spin for a fixed time period
   * (defined below) to allow SRCU readers to exit their read-side critical
   * sections.  If there are still some readers after 10 microseconds,
   * we repeatedly block for 1-millisecond time periods.  This approach
   * has done well in testing, so there is no need for a config parameter.
   */

  #define SRCU_RETRY_CHECK_DELAY		5

  #define SYNCHRONIZE_SRCU_TRYCOUNT	2
  #define SYNCHRONIZE_SRCU_EXP_TRYCOUNT	12

  /*

   * @@@ Wait until all pre-existing readers complete.  Such readers

   * will have used the index specified by "idx".

   * the caller should ensures the ->completed is not changed while checking
   * and idx = (->completed & 1) ^ 1

   */

  static bool try_check_zero(struct srcu_struct *sp, int idx, int trycount)

  {

  	for (;;) {
  		if (srcu_readers_active_idx_check(sp, idx))
  			return true;
  		if (--trycount <= 0)
  			return false;
  		udelay(SRCU_RETRY_CHECK_DELAY);

  	}

  }

  /*
   * Increment the ->completed counter so that future SRCU readers will
   * use the other rank of the ->c[] and ->seq[] arrays.  This allows
   * us to wait for pre-existing readers in a starvation-free manner.
   */

  static void srcu_flip(struct srcu_struct *sp)

  {

  	sp->completed++;

  }
  
  /*

   * Enqueue an SRCU callback on the specified srcu_struct structure,
   * initiating grace-period processing if it is not already running.
   */
  void call_srcu(struct srcu_struct *sp, struct rcu_head *head,
  		void (*func)(struct rcu_head *head))
  {
  	unsigned long flags;
  
  	head->next = NULL;
  	head->func = func;
  	spin_lock_irqsave(&sp->queue_lock, flags);
  	rcu_batch_queue(&sp->batch_queue, head);
  	if (!sp->running) {
  		sp->running = true;

  		schedule_delayed_work(&sp->work, 0);

  	}
  	spin_unlock_irqrestore(&sp->queue_lock, flags);
  }
  EXPORT_SYMBOL_GPL(call_srcu);
  
  struct rcu_synchronize {
  	struct rcu_head head;
  	struct completion completion;
  };
  
  /*
   * Awaken the corresponding synchronize_srcu() instance now that a
   * grace period has elapsed.
   */
  static void wakeme_after_rcu(struct rcu_head *head)
  {
  	struct rcu_synchronize *rcu;
  
  	rcu = container_of(head, struct rcu_synchronize, head);
  	complete(&rcu->completion);
  }
  
  static void srcu_advance_batches(struct srcu_struct *sp, int trycount);
  static void srcu_reschedule(struct srcu_struct *sp);
  
  /*

   * Helper function for synchronize_srcu() and synchronize_srcu_expedited().

   */

  static void __synchronize_srcu(struct srcu_struct *sp, int trycount)

  {

  	struct rcu_synchronize rcu;
  	struct rcu_head *head = &rcu.head;
  	bool done = false;

  	rcu_lockdep_assert(!lock_is_held(&sp->dep_map) &&
  			   !lock_is_held(&rcu_bh_lock_map) &&
  			   !lock_is_held(&rcu_lock_map) &&
  			   !lock_is_held(&rcu_sched_lock_map),
  			   "Illegal synchronize_srcu() in same-type SRCU (or RCU) read-side critical section");

  	might_sleep();

  	init_completion(&rcu.completion);
  
  	head->next = NULL;
  	head->func = wakeme_after_rcu;
  	spin_lock_irq(&sp->queue_lock);
  	if (!sp->running) {
  		/* steal the processing owner */
  		sp->running = true;
  		rcu_batch_queue(&sp->batch_check0, head);
  		spin_unlock_irq(&sp->queue_lock);
  
  		srcu_advance_batches(sp, trycount);
  		if (!rcu_batch_empty(&sp->batch_done)) {
  			BUG_ON(sp->batch_done.head != head);
  			rcu_batch_dequeue(&sp->batch_done);
  			done = true;
  		}
  		/* give the processing owner to work_struct */
  		srcu_reschedule(sp);
  	} else {
  		rcu_batch_queue(&sp->batch_queue, head);
  		spin_unlock_irq(&sp->queue_lock);
  	}

  	if (!done)
  		wait_for_completion(&rcu.completion);

  }
  
  /**

   * synchronize_srcu - wait for prior SRCU read-side critical-section completion
   * @sp: srcu_struct with which to synchronize.
   *

   * Wait for the count to drain to zero of both indexes. To avoid the
   * possible starvation of synchronize_srcu(), it waits for the count of
   * the index=((->completed & 1) ^ 1) to drain to zero at first,
   * and then flip the completed and wait for the count of the other index.
   *
   * Can block; must be called from process context.

   *
   * Note that it is illegal to call synchronize_srcu() from the corresponding
   * SRCU read-side critical section; doing so will result in deadlock.
   * However, it is perfectly legal to call synchronize_srcu() on one
   * srcu_struct from some other srcu_struct's read-side critical section.
   */
  void synchronize_srcu(struct srcu_struct *sp)
  {

  	__synchronize_srcu(sp, rcu_expedited
  			   ? SYNCHRONIZE_SRCU_EXP_TRYCOUNT
  			   : SYNCHRONIZE_SRCU_TRYCOUNT);

  }
  EXPORT_SYMBOL_GPL(synchronize_srcu);
  
  /**

   * synchronize_srcu_expedited - Brute-force SRCU grace period

   * @sp: srcu_struct with which to synchronize.
   *

   * Wait for an SRCU grace period to elapse, but be more aggressive about
   * spinning rather than blocking when waiting.

   *

   * Note that it is also illegal to call synchronize_srcu_expedited()
   * from the corresponding SRCU read-side critical section;
   * doing so will result in deadlock.  However, it is perfectly legal
   * to call synchronize_srcu_expedited() on one srcu_struct from some
   * other srcu_struct's read-side critical section, as long as

   * the resulting graph of srcu_structs is acyclic.

   */
  void synchronize_srcu_expedited(struct srcu_struct *sp)
  {

  	__synchronize_srcu(sp, SYNCHRONIZE_SRCU_EXP_TRYCOUNT);

  }
  EXPORT_SYMBOL_GPL(synchronize_srcu_expedited);
  
  /**

   * srcu_barrier - Wait until all in-flight call_srcu() callbacks complete.
   */
  void srcu_barrier(struct srcu_struct *sp)
  {
  	synchronize_srcu(sp);
  }
  EXPORT_SYMBOL_GPL(srcu_barrier);
  
  /**

   * srcu_batches_completed - return batches completed.
   * @sp: srcu_struct on which to report batch completion.
   *
   * Report the number of batches, correlated with, but not necessarily
   * precisely the same as, the number of grace periods that have elapsed.
   */

  long srcu_batches_completed(struct srcu_struct *sp)
  {
  	return sp->completed;
  }

  EXPORT_SYMBOL_GPL(srcu_batches_completed);

  
  #define SRCU_CALLBACK_BATCH	10
  #define SRCU_INTERVAL		1
  
  /*
   * Move any new SRCU callbacks to the first stage of the SRCU grace
   * period pipeline.
   */
  static void srcu_collect_new(struct srcu_struct *sp)
  {
  	if (!rcu_batch_empty(&sp->batch_queue)) {
  		spin_lock_irq(&sp->queue_lock);
  		rcu_batch_move(&sp->batch_check0, &sp->batch_queue);
  		spin_unlock_irq(&sp->queue_lock);
  	}
  }
  
  /*
   * Core SRCU state machine.  Advance callbacks from ->batch_check0 to
   * ->batch_check1 and then to ->batch_done as readers drain.
   */
  static void srcu_advance_batches(struct srcu_struct *sp, int trycount)
  {
  	int idx = 1 ^ (sp->completed & 1);
  
  	/*
  	 * Because readers might be delayed for an extended period after
  	 * fetching ->completed for their index, at any point in time there
  	 * might well be readers using both idx=0 and idx=1.  We therefore
  	 * need to wait for readers to clear from both index values before
  	 * invoking a callback.
  	 */
  
  	if (rcu_batch_empty(&sp->batch_check0) &&
  	    rcu_batch_empty(&sp->batch_check1))
  		return; /* no callbacks need to be advanced */
  
  	if (!try_check_zero(sp, idx, trycount))
  		return; /* failed to advance, will try after SRCU_INTERVAL */
  
  	/*
  	 * The callbacks in ->batch_check1 have already done with their
  	 * first zero check and flip back when they were enqueued on
  	 * ->batch_check0 in a previous invocation of srcu_advance_batches().
  	 * (Presumably try_check_zero() returned false during that
  	 * invocation, leaving the callbacks stranded on ->batch_check1.)
  	 * They are therefore ready to invoke, so move them to ->batch_done.
  	 */
  	rcu_batch_move(&sp->batch_done, &sp->batch_check1);
  
  	if (rcu_batch_empty(&sp->batch_check0))
  		return; /* no callbacks need to be advanced */
  	srcu_flip(sp);
  
  	/*
  	 * The callbacks in ->batch_check0 just finished their
  	 * first check zero and flip, so move them to ->batch_check1
  	 * for future checking on the other idx.
  	 */
  	rcu_batch_move(&sp->batch_check1, &sp->batch_check0);
  
  	/*
  	 * SRCU read-side critical sections are normally short, so check
  	 * at least twice in quick succession after a flip.
  	 */
  	trycount = trycount < 2 ? 2 : trycount;
  	if (!try_check_zero(sp, idx^1, trycount))
  		return; /* failed to advance, will try after SRCU_INTERVAL */
  
  	/*
  	 * The callbacks in ->batch_check1 have now waited for all
  	 * pre-existing readers using both idx values.  They are therefore
  	 * ready to invoke, so move them to ->batch_done.
  	 */
  	rcu_batch_move(&sp->batch_done, &sp->batch_check1);
  }
  
  /*
   * Invoke a limited number of SRCU callbacks that have passed through
   * their grace period.  If there are more to do, SRCU will reschedule
   * the workqueue.
   */
  static void srcu_invoke_callbacks(struct srcu_struct *sp)
  {
  	int i;
  	struct rcu_head *head;
  
  	for (i = 0; i < SRCU_CALLBACK_BATCH; i++) {
  		head = rcu_batch_dequeue(&sp->batch_done);
  		if (!head)
  			break;
  		local_bh_disable();
  		head->func(head);
  		local_bh_enable();
  	}
  }
  
  /*
   * Finished one round of SRCU grace period.  Start another if there are
   * more SRCU callbacks queued, otherwise put SRCU into not-running state.
   */
  static void srcu_reschedule(struct srcu_struct *sp)
  {
  	bool pending = true;
  
  	if (rcu_batch_empty(&sp->batch_done) &&
  	    rcu_batch_empty(&sp->batch_check1) &&
  	    rcu_batch_empty(&sp->batch_check0) &&
  	    rcu_batch_empty(&sp->batch_queue)) {
  		spin_lock_irq(&sp->queue_lock);
  		if (rcu_batch_empty(&sp->batch_done) &&
  		    rcu_batch_empty(&sp->batch_check1) &&
  		    rcu_batch_empty(&sp->batch_check0) &&
  		    rcu_batch_empty(&sp->batch_queue)) {
  			sp->running = false;
  			pending = false;
  		}
  		spin_unlock_irq(&sp->queue_lock);
  	}
  
  	if (pending)

  		schedule_delayed_work(&sp->work, SRCU_INTERVAL);

  }
  
  /*
   * This is the work-queue function that handles SRCU grace periods.
   */

  void process_srcu(struct work_struct *work)

  {
  	struct srcu_struct *sp;
  
  	sp = container_of(work, struct srcu_struct, work.work);
  
  	srcu_collect_new(sp);
  	srcu_advance_batches(sp, 1);
  	srcu_invoke_callbacks(sp);
  	srcu_reschedule(sp);
  }

  EXPORT_SYMBOL_GPL(process_srcu);