RFC: FROMLIST: locking/percpu-rwsem: Optimize readers and reduce global impact

struct percpu_rw_semaphore {

	struct rcu_sync		rss;

	unsigned int __percpu	*fast_read_ctr;

	unsigned int __percpu	*read_count;

	struct rw_semaphore	rw_sem;

	atomic_t		slow_read_ctr;

	wait_queue_head_t	write_waitq;

	wait_queue_head_t	writer;

	int			readers_block;

};

extern void percpu_down_read(struct percpu_rw_semaphore *);

extern int  percpu_down_read_trylock(struct percpu_rw_semaphore *);

extern void percpu_up_read(struct percpu_rw_semaphore *);

extern int __percpu_down_read(struct percpu_rw_semaphore *, int);

extern void __percpu_up_read(struct percpu_rw_semaphore *);

static inline void percpu_down_read(struct percpu_rw_semaphore *sem)

{

	might_sleep();

	rwsem_acquire_read(&sem->rw_sem.dep_map, 0, 0, _RET_IP_);

	preempt_disable();

	/*

	 * We are in an RCU-sched read-side critical section, so the writer

	 * cannot both change sem->state from readers_fast and start checking

	 * counters while we are here. So if we see !sem->state, we know that

	 * the writer won't be checking until we're past the preempt_enable()

	 * and that one the synchronize_sched() is done, the writer will see

	 * anything we did within this RCU-sched read-size critical section.

	 */

	__this_cpu_inc(*sem->read_count);

	if (unlikely(!rcu_sync_is_idle(&sem->rss)))

		__percpu_down_read(sem, false); /* Unconditional memory barrier */

	preempt_enable();

	/*

	 * The barrier() from preempt_enable() prevents the compiler from

	 * bleeding the critical section out.

	 */

}

static inline int percpu_down_read_trylock(struct percpu_rw_semaphore *sem)

{

	int ret = 1;

	preempt_disable();

	/*

	 * Same as in percpu_down_read().

	 */

	__this_cpu_inc(*sem->read_count);

	if (unlikely(!rcu_sync_is_idle(&sem->rss)))

		ret = __percpu_down_read(sem, true); /* Unconditional memory barrier */

	preempt_enable();

	/*

	 * The barrier() from preempt_enable() prevents the compiler from

	 * bleeding the critical section out.

	 */

	if (ret)

		rwsem_acquire_read(&sem->rw_sem.dep_map, 0, 1, _RET_IP_);

	return ret;

}

static inline void percpu_up_read(struct percpu_rw_semaphore *sem)

{

	/*

	 * The barrier() in preempt_disable() prevents the compiler from

	 * bleeding the critical section out.

	 */

	preempt_disable();

	/*

	 * Same as in percpu_down_read().

	 */

	if (likely(rcu_sync_is_idle(&sem->rss)))

		__this_cpu_dec(*sem->read_count);

	else

		__percpu_up_read(sem); /* Unconditional memory barrier */

	preempt_enable();

	rwsem_release(&sem->rw_sem.dep_map, 1, _RET_IP_);

}

extern void percpu_down_write(struct percpu_rw_semaphore *);

extern void percpu_up_write(struct percpu_rw_semaphore *);

extern int __percpu_init_rwsem(struct percpu_rw_semaphore *,

				const char *, struct lock_class_key *);

extern void percpu_free_rwsem(struct percpu_rw_semaphore *);

#define percpu_init_rwsem(brw)	\

#define percpu_init_rwsem(sem)					\

({								\

	static struct lock_class_key rwsem_key;			\

	__percpu_init_rwsem(brw, #brw, &rwsem_key);		\

	__percpu_init_rwsem(sem, #sem, &rwsem_key);		\

})

#define percpu_rwsem_is_held(sem) lockdep_is_held(&(sem)->rw_sem)

static inline void percpu_rwsem_release(struct percpu_rw_semaphore *sem,

#include <linux/sched.h>

#include <linux/errno.h>

int __percpu_init_rwsem(struct percpu_rw_semaphore *brw,

int __percpu_init_rwsem(struct percpu_rw_semaphore *sem,

			const char *name, struct lock_class_key *rwsem_key)

{

	brw->fast_read_ctr = alloc_percpu(int);

	if (unlikely(!brw->fast_read_ctr))

	sem->read_count = alloc_percpu(int);

	if (unlikely(!sem->read_count))

		return -ENOMEM;

	/* ->rw_sem represents the whole percpu_rw_semaphore for lockdep */

	__init_rwsem(&brw->rw_sem, name, rwsem_key);

	rcu_sync_init(&brw->rss, RCU_SCHED_SYNC);

	atomic_set(&brw->slow_read_ctr, 0);

	init_waitqueue_head(&brw->write_waitq);

	rcu_sync_init(&sem->rss, RCU_SCHED_SYNC);

	__init_rwsem(&sem->rw_sem, name, rwsem_key);

	init_waitqueue_head(&sem->writer);

	sem->readers_block = 0;

	return 0;

}

EXPORT_SYMBOL_GPL(__percpu_init_rwsem);

void percpu_free_rwsem(struct percpu_rw_semaphore *brw)

void percpu_free_rwsem(struct percpu_rw_semaphore *sem)

{

	/*

	 * XXX: temporary kludge. The error path in alloc_super()

	 * assumes that percpu_free_rwsem() is safe after kzalloc().

	 */

	if (!brw->fast_read_ctr)

	if (!sem->read_count)

		return;

	rcu_sync_dtor(&brw->rss);

	free_percpu(brw->fast_read_ctr);

	brw->fast_read_ctr = NULL; /* catch use after free bugs */

	rcu_sync_dtor(&sem->rss);

	free_percpu(sem->read_count);

	sem->read_count = NULL; /* catch use after free bugs */

}

EXPORT_SYMBOL_GPL(percpu_free_rwsem);

int __percpu_down_read(struct percpu_rw_semaphore *sem, int try)

{

	/*

 * This is the fast-path for down_read/up_read. If it succeeds we rely

 * on the barriers provided by rcu_sync_enter/exit; see the comments in

 * percpu_down_write() and percpu_up_write().

	 * Due to having preemption disabled the decrement happens on

	 * the same CPU as the increment, avoiding the

	 * increment-on-one-CPU-and-decrement-on-another problem.

	 *

	 * If the reader misses the writer's assignment of readers_block, then

	 * the writer is guaranteed to see the reader's increment.

	 *

 * If this helper fails the callers rely on the normal rw_semaphore and

 * atomic_dec_and_test(), so in this case we have the necessary barriers.

	 * Conversely, any readers that increment their sem->read_count after

	 * the writer looks are guaranteed to see the readers_block value,

	 * which in turn means that they are guaranteed to immediately

	 * decrement their sem->read_count, so that it doesn't matter that the

	 * writer missed them.

	 */

static bool update_fast_ctr(struct percpu_rw_semaphore *brw, unsigned int val)

{

	bool success;

	preempt_disable();

	success = rcu_sync_is_idle(&brw->rss);

	if (likely(success))

		__this_cpu_add(*brw->fast_read_ctr, val);

	preempt_enable();

	smp_mb(); /* A matches D */

	return success;

}

	/*

	 * If !readers_block the critical section starts here, matched by the

	 * release in percpu_up_write().

	 */

	if (likely(!smp_load_acquire(&sem->readers_block)))

		return 1;

	/*

 * Like the normal down_read() this is not recursive, the writer can

 * come after the first percpu_down_read() and create the deadlock.

 *

 * Note: returns with lock_is_held(brw->rw_sem) == T for lockdep,

 * percpu_up_read() does rwsem_release(). This pairs with the usage

 * of ->rw_sem in percpu_down/up_write().

	 * Per the above comment; we still have preemption disabled and

	 * will thus decrement on the same CPU as we incremented.

	 */

void percpu_down_read(struct percpu_rw_semaphore *brw)

{

	might_sleep();

	rwsem_acquire_read(&brw->rw_sem.dep_map, 0, 0, _RET_IP_);

	__percpu_up_read(sem);

	if (likely(update_fast_ctr(brw, +1)))

		return;

	if (try)

		return 0;

	/* Avoid rwsem_acquire_read() and rwsem_release() */

	__down_read(&brw->rw_sem);

	atomic_inc(&brw->slow_read_ctr);

	__up_read(&brw->rw_sem);

}

EXPORT_SYMBOL_GPL(percpu_down_read);

	/*

	 * We either call schedule() in the wait, or we'll fall through

	 * and reschedule on the preempt_enable() in percpu_down_read().

	 */

	preempt_enable_no_resched();

int percpu_down_read_trylock(struct percpu_rw_semaphore *brw)

{

	if (unlikely(!update_fast_ctr(brw, +1))) {

		if (!__down_read_trylock(&brw->rw_sem))

			return 0;

		atomic_inc(&brw->slow_read_ctr);

		__up_read(&brw->rw_sem);

	}

	/*

	 * Avoid lockdep for the down/up_read() we already have them.

	 */

	__down_read(&sem->rw_sem);

	this_cpu_inc(*sem->read_count);

	__up_read(&sem->rw_sem);

	rwsem_acquire_read(&brw->rw_sem.dep_map, 0, 1, _RET_IP_);

	preempt_disable();

	return 1;

}

EXPORT_SYMBOL_GPL(__percpu_down_read);

void percpu_up_read(struct percpu_rw_semaphore *brw)

void __percpu_up_read(struct percpu_rw_semaphore *sem)

{

	rwsem_release(&brw->rw_sem.dep_map, 1, _RET_IP_);

	if (likely(update_fast_ctr(brw, -1)))

		return;

	smp_mb(); /* B matches C */

	/*

	 * In other words, if they see our decrement (presumably to aggregate

	 * zero, as that is the only time it matters) they will also see our

	 * critical section.

	 */

	__this_cpu_dec(*sem->read_count);

	/* false-positive is possible but harmless */

	if (atomic_dec_and_test(&brw->slow_read_ctr))

		wake_up_all(&brw->write_waitq);

	/* Prod writer to recheck readers_active */

	wake_up(&sem->writer);

}

EXPORT_SYMBOL_GPL(percpu_up_read);

EXPORT_SYMBOL_GPL(__percpu_up_read);

#define per_cpu_sum(var)						\

({									\

	typeof(var) __sum = 0;						\

	int cpu;							\

	compiletime_assert_atomic_type(__sum);				\

	for_each_possible_cpu(cpu)					\

		__sum += per_cpu(var, cpu);				\

	__sum;								\

})

static int clear_fast_ctr(struct percpu_rw_semaphore *brw)

/*

 * Return true if the modular sum of the sem->read_count per-CPU variable is

 * zero.  If this sum is zero, then it is stable due to the fact that if any

 * newly arriving readers increment a given counter, they will immediately

 * decrement that same counter.

 */

static bool readers_active_check(struct percpu_rw_semaphore *sem)

{

	unsigned int sum = 0;

	int cpu;

	if (per_cpu_sum(*sem->read_count) != 0)

		return false;

	for_each_possible_cpu(cpu) {

		sum += per_cpu(*brw->fast_read_ctr, cpu);

		per_cpu(*brw->fast_read_ctr, cpu) = 0;

	}

	/*

	 * If we observed the decrement; ensure we see the entire critical

	 * section.

	 */

	smp_mb(); /* C matches B */

	return sum;

	return true;

}

void percpu_down_write(struct percpu_rw_semaphore *brw)

void percpu_down_write(struct percpu_rw_semaphore *sem)

{

	/* Notify readers to take the slow path. */

	rcu_sync_enter(&sem->rss);

	down_write(&sem->rw_sem);

	/*

	 * Make rcu_sync_is_idle() == F and thus disable the fast-path in

	 * percpu_down_read() and percpu_up_read(), and wait for gp pass.

	 *

	 * The latter synchronises us with the preceding readers which used

	 * the fast-past, so we can not miss the result of __this_cpu_add()

	 * or anything else inside their criticial sections.

	 * Notify new readers to block; up until now, and thus throughout the

	 * longish rcu_sync_enter() above, new readers could still come in.

	 */

	rcu_sync_enter(&brw->rss);

	WRITE_ONCE(sem->readers_block, 1);

	/* exclude other writers, and block the new readers completely */

	down_write(&brw->rw_sem);

	smp_mb(); /* D matches A */

	/* nobody can use fast_read_ctr, move its sum into slow_read_ctr */

	atomic_add(clear_fast_ctr(brw), &brw->slow_read_ctr);

	/*

	 * If they don't see our writer of readers_block, then we are

	 * guaranteed to see their sem->read_count increment, and therefore

	 * will wait for them.

	 */

	/* wait for all readers to complete their percpu_up_read() */

	wait_event(brw->write_waitq, !atomic_read(&brw->slow_read_ctr));

	/* Wait for all now active readers to complete. */

	wait_event(sem->writer, readers_active_check(sem));

}

EXPORT_SYMBOL_GPL(percpu_down_write);

void percpu_up_write(struct percpu_rw_semaphore *brw)

void percpu_up_write(struct percpu_rw_semaphore *sem)

{

	/* release the lock, but the readers can't use the fast-path */

	up_write(&brw->rw_sem);

	/*

	 * Enable the fast-path in percpu_down_read() and percpu_up_read()

	 * but only after another gp pass; this adds the necessary barrier

	 * to ensure the reader can't miss the changes done by us.

	 * Signal the writer is done, no fast path yet.

	 *

	 * One reason that we cannot just immediately flip to readers_fast is

	 * that new readers might fail to see the results of this writer's

	 * critical section.

	 *

	 * Therefore we force it through the slow path which guarantees an

	 * acquire and thereby guarantees the critical section's consistency.

	 */

	smp_store_release(&sem->readers_block, 0);

	/*

	 * Release the write lock, this will allow readers back in the game.

	 */

	up_write(&sem->rw_sem);

	/*

	 * Once this completes (at least one RCU-sched grace period hence) the

	 * reader fast path will be available again. Safe to use outside the

	 * exclusive write lock because its counting.

	 */

	rcu_sync_exit(&brw->rss);

	rcu_sync_exit(&sem->rss);

}

EXPORT_SYMBOL_GPL(percpu_up_write);