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#include <linux/atomic.h>
#include <linux/rwsem.h>
#include <linux/percpu.h>
#include <linux/wait.h>
#include <linux/lockdep.h>
#include <linux/percpu-rwsem.h>
#include <linux/rcupdate.h>
#include <linux/sched.h>
#include <linux/errno.h>

int __percpu_init_rwsem(struct percpu_rw_semaphore *brw,
			const char *name, struct lock_class_key *rwsem_key)
	brw->fast_read_ctr = alloc_percpu(int);
	if (unlikely(!brw->fast_read_ctr))
		return -ENOMEM;

	/* ->rw_sem represents the whole percpu_rw_semaphore for lockdep */
	__init_rwsem(&brw->rw_sem, name, rwsem_key);
	atomic_set(&brw->write_ctr, 0);
	atomic_set(&brw->slow_read_ctr, 0);
	return 0;

void percpu_free_rwsem(struct percpu_rw_semaphore *brw)
	brw->fast_read_ctr = NULL; /* catch use after free bugs */

 * This is the fast-path for down_read/up_read, it only needs to ensure
 * there is no pending writer (atomic_read(write_ctr) == 0) and inc/dec the
 * fast per-cpu counter. The writer uses synchronize_sched_expedited() to
 * serialize with the preempt-disabled section below.
 * The nontrivial part is that we should guarantee acquire/release semantics
 * in case when
 *	R_W: down_write() comes after up_read(), the writer should see all
 *	     changes done by the reader
 * or
 *	W_R: down_read() comes after up_write(), the reader should see all
 *	     changes done by the writer
 * 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.
 * But if it succeeds we do not have any barriers, atomic_read(write_ctr) or
 * __this_cpu_add() below can be reordered with any LOAD/STORE done by the
 * reader inside the critical section. See the comments in down_write and
 * up_write below.
static bool update_fast_ctr(struct percpu_rw_semaphore *brw, unsigned int val)
	bool success = false;

	if (likely(!atomic_read(&brw->write_ctr))) {
		__this_cpu_add(*brw->fast_read_ctr, val);
		success = true;

	return success;

 * 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().
void percpu_down_read(struct percpu_rw_semaphore *brw)
	if (likely(update_fast_ctr(brw, +1))) {
		rwsem_acquire_read(&brw->rw_sem.dep_map, 0, 0, _RET_IP_);

	/* avoid up_read()->rwsem_release() */

void percpu_up_read(struct percpu_rw_semaphore *brw)
	rwsem_release(&brw->rw_sem.dep_map, 1, _RET_IP_);

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

	/* false-positive is possible but harmless */
	if (atomic_dec_and_test(&brw->slow_read_ctr))

static int clear_fast_ctr(struct percpu_rw_semaphore *brw)
	unsigned int sum = 0;
	int cpu;

	for_each_possible_cpu(cpu) {
		sum += per_cpu(*brw->fast_read_ctr, cpu);
		per_cpu(*brw->fast_read_ctr, cpu) = 0;

	return sum;

 * A writer increments ->write_ctr to force the readers to switch to the
 * slow mode, note the atomic_read() check in update_fast_ctr().
 * After that the readers can only inc/dec the slow ->slow_read_ctr counter,
 * ->fast_read_ctr is stable. Once the writer moves its sum into the slow
 * counter it represents the number of active readers.
 * Finally the writer takes ->rw_sem for writing and blocks the new readers,
 * then waits until the slow counter becomes zero.
void percpu_down_write(struct percpu_rw_semaphore *brw)
	/* tell update_fast_ctr() there is a pending writer */
	 * 1. Ensures that write_ctr != 0 is visible to any down_read/up_read
	 *    so that update_fast_ctr() can't succeed.
	 * 2. Ensures we see the result of every previous this_cpu_add() in
	 *    update_fast_ctr().
	 * 3. Ensures that if any reader has exited its critical section via
	 *    fast-path, it executes a full memory barrier before we return.
	 *    See R_W case in the comment above update_fast_ctr().

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

	/* nobody can use fast_read_ctr, move its sum into slow_read_ctr */
	atomic_add(clear_fast_ctr(brw), &brw->slow_read_ctr);

	/* wait for all readers to complete their percpu_up_read() */
	wait_event(brw->write_waitq, !atomic_read(&brw->slow_read_ctr));

void percpu_up_write(struct percpu_rw_semaphore *brw)
	/* release the lock, but the readers can't use the fast-path */
	 * Insert the barrier before the next fast-path in down_read,
	 * see W_R case in the comment above update_fast_ctr().
	/* the last writer unblocks update_fast_ctr() */