kernel/srcu.c

/*
 * 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>

/*
 * 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);
	}
}

/* single-thread state-machine */
static void process_srcu(struct work_struct *work);

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)
{
	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 = rcu_dereference_index_check(sp->completed,
					  rcu_read_lock_sched_held()) & 0x1;
	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)
{
	preempt_disable();
	smp_mb(); /* C */  /* Avoid leaking the critical section. */
	ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->c[idx]) -= 1;
	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 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");

	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.
 *
 * 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_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 illegal to call this function while holding any lock
 * that is acquired by a CPU-hotplug notifier.  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.
 */
static 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);
}