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

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

  /*
   * srcu_readers_active_idx -- returns approximate number of readers
   *	active on the specified rank of per-CPU counters.
   */
  
  static int srcu_readers_active_idx(struct srcu_struct *sp, int idx)
  {
  	int cpu;
  	int sum;
  
  	sum = 0;
  	for_each_possible_cpu(cpu)
  		sum += per_cpu_ptr(sp->per_cpu_ref, cpu)->c[idx];
  	return sum;
  }
  
  /**
   * 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)

  {
  	return srcu_readers_active_idx(sp, 0) + srcu_readers_active_idx(sp, 1);
  }
  
  /**
   * 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)
  {
  	int sum;
  
  	sum = srcu_readers_active(sp);
  	WARN_ON(sum);  /* Leakage unless caller handles error. */
  	if (sum != 0)
  		return;
  	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;
  
  	preempt_disable();
  	idx = sp->completed & 0x1;
  	barrier();  /* ensure compiler looks -once- at sp->completed. */
  	per_cpu_ptr(sp->per_cpu_ref, smp_processor_id())->c[idx]++;
  	srcu_barrier();  /* ensure compiler won't misorder critical section. */
  	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)

  {
  	preempt_disable();
  	srcu_barrier();  /* ensure compiler won't misorder critical section. */
  	per_cpu_ptr(sp->per_cpu_ref, smp_processor_id())->c[idx]--;
  	preempt_enable();
  }

  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 SYNCHRONIZE_SRCU_READER_DELAY 10
  
  /*

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

   */

  static void __synchronize_srcu(struct srcu_struct *sp, void (*sync_func)(void))

  {
  	int idx;
  
  	idx = sp->completed;
  	mutex_lock(&sp->mutex);
  
  	/*
  	 * Check to see if someone else did the work for us while we were
  	 * waiting to acquire the lock.  We need -two- advances of
  	 * the counter, not just one.  If there was but one, we might have
  	 * shown up -after- our helper's first synchronize_sched(), thus
  	 * having failed to prevent CPU-reordering races with concurrent
  	 * srcu_read_unlock()s on other CPUs (see comment below).  So we
  	 * either (1) wait for two or (2) supply the second ourselves.
  	 */
  
  	if ((sp->completed - idx) >= 2) {
  		mutex_unlock(&sp->mutex);
  		return;
  	}

  	sync_func();  /* Force memory barrier on all CPUs. */

  
  	/*
  	 * The preceding synchronize_sched() ensures that any CPU that
  	 * sees the new value of sp->completed will also see any preceding
  	 * changes to data structures made by this CPU.  This prevents
  	 * some other CPU from reordering the accesses in its SRCU
  	 * read-side critical section to precede the corresponding
  	 * srcu_read_lock() -- ensuring that such references will in
  	 * fact be protected.
  	 *
  	 * So it is now safe to do the flip.
  	 */
  
  	idx = sp->completed & 0x1;
  	sp->completed++;

  	sync_func();  /* Force memory barrier on all CPUs. */

  
  	/*
  	 * At this point, because of the preceding synchronize_sched(),
  	 * all srcu_read_lock() calls using the old counters have completed.
  	 * Their corresponding critical sections might well be still
  	 * executing, but the srcu_read_lock() primitives themselves

  	 * will have finished executing.  We initially give readers
  	 * an arbitrarily chosen 10 microseconds to get out of their
  	 * SRCU read-side critical sections, then loop waiting 1/HZ

  	 * seconds per iteration.  The 10-microsecond value has done
  	 * very well in testing.

  	 */

  	if (srcu_readers_active_idx(sp, idx))

  		udelay(SYNCHRONIZE_SRCU_READER_DELAY);

  	while (srcu_readers_active_idx(sp, idx))
  		schedule_timeout_interruptible(1);

  	sync_func();  /* Force memory barrier on all CPUs. */

  
  	/*
  	 * The preceding synchronize_sched() forces all srcu_read_unlock()
  	 * primitives that were executing concurrently with the preceding
  	 * for_each_possible_cpu() loop to have completed by this point.
  	 * More importantly, it also forces the corresponding SRCU read-side
  	 * critical sections to have also completed, and the corresponding
  	 * references to SRCU-protected data items to be dropped.
  	 *
  	 * Note:
  	 *
  	 *	Despite what you might think at first glance, the
  	 *	preceding synchronize_sched() -must- be within the
  	 *	critical section ended by the following mutex_unlock().
  	 *	Otherwise, a task taking the early exit can race
  	 *	with a srcu_read_unlock(), which might have executed
  	 *	just before the preceding srcu_readers_active() check,
  	 *	and whose CPU might have reordered the srcu_read_unlock()
  	 *	with the preceding critical section.  In this case, there
  	 *	is nothing preventing the synchronize_sched() task that is
  	 *	taking the early exit from freeing a data structure that
  	 *	is still being referenced (out of order) by the task
  	 *	doing the srcu_read_unlock().
  	 *
  	 *	Alternatively, the comparison with "2" on the early exit
  	 *	could be changed to "3", but this increases synchronize_srcu()
  	 *	latency for bulk loads.  So the current code is preferred.
  	 */
  
  	mutex_unlock(&sp->mutex);
  }
  
  /**

   * synchronize_srcu - wait for prior SRCU read-side critical-section completion
   * @sp: srcu_struct with which to synchronize.
   *
   * Flip the completed counter, and wait for the old count to drain to zero.
   * As with classic RCU, the updater must use some separate means of
   * synchronizing concurrent updates.  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, synchronize_sched);
  }
  EXPORT_SYMBOL_GPL(synchronize_srcu);
  
  /**
   * synchronize_srcu_expedited - like synchronize_srcu, but less patient
   * @sp: srcu_struct with which to synchronize.
   *
   * Flip the completed counter, and wait for the old count to drain to zero.
   * As with classic RCU, the updater must use some separate means of
   * synchronizing concurrent updates.  Can block; must be called from
   * process context.
   *
   * Note that it is 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.
   */
  void synchronize_srcu_expedited(struct srcu_struct *sp)
  {
  	__synchronize_srcu(sp, synchronize_sched_expedited);
  }
  EXPORT_SYMBOL_GPL(synchronize_srcu_expedited);
  
  /**

   * 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);