btrfs: Implement DREW lock

#include "extent_map.h"

#include "async-thread.h"

#include "block-rsv.h"

#include "locking.h"

struct btrfs_trans_handle;

struct btrfs_transaction;

	}

	return eb;

}

/*

 * DREW locks

 * ==========

 *

 * DREW stands for double-reader-writer-exclusion lock. It's used in situation

 * where you want to provide A-B exclusion but not AA or BB.

 *

 * Currently implementation gives more priority to reader. If a reader and a

 * writer both race to acquire their respective sides of the lock the writer

 * would yield its lock as soon as it detects a concurrent reader. Additionally

 * if there are pending readers no new writers would be allowed to come in and

 * acquire the lock.

 */

int btrfs_drew_lock_init(struct btrfs_drew_lock *lock)

{

	int ret;

	ret = percpu_counter_init(&lock->writers, 0, GFP_KERNEL);

	if (ret)

		return ret;

	atomic_set(&lock->readers, 0);

	init_waitqueue_head(&lock->pending_readers);

	init_waitqueue_head(&lock->pending_writers);

	return 0;

}

void btrfs_drew_lock_destroy(struct btrfs_drew_lock *lock)

{

	percpu_counter_destroy(&lock->writers);

}

/* Return true if acquisition is successful, false otherwise */

bool btrfs_drew_try_write_lock(struct btrfs_drew_lock *lock)

{

	if (atomic_read(&lock->readers))

		return false;

	percpu_counter_inc(&lock->writers);

	/* Ensure writers count is updated before we check for pending readers */

	smp_mb();

	if (atomic_read(&lock->readers)) {

		btrfs_drew_write_unlock(lock);

		return false;

	}

	return true;

}

void btrfs_drew_write_lock(struct btrfs_drew_lock *lock)

{

	while (true) {

		if (btrfs_drew_try_write_lock(lock))

			return;

		wait_event(lock->pending_writers, !atomic_read(&lock->readers));

	}

}

void btrfs_drew_write_unlock(struct btrfs_drew_lock *lock)

{

	percpu_counter_dec(&lock->writers);

	cond_wake_up(&lock->pending_readers);

}

void btrfs_drew_read_lock(struct btrfs_drew_lock *lock)

{

	atomic_inc(&lock->readers);

	/*

	 * Ensure the pending reader count is perceieved BEFORE this reader

	 * goes to sleep in case of active writers. This guarantees new writers

	 * won't be allowed and that the current reader will be woken up when

	 * the last active writer finishes its jobs.

	 */

	smp_mb__after_atomic();

	wait_event(lock->pending_readers,

		   percpu_counter_sum(&lock->writers) == 0);

}

void btrfs_drew_read_unlock(struct btrfs_drew_lock *lock)

{

	/*

	 * atomic_dec_and_test implies a full barrier, so woken up writers

	 * are guaranteed to see the decrement

	 */

	if (atomic_dec_and_test(&lock->readers))

		wake_up(&lock->pending_writers);

}

#ifndef BTRFS_LOCKING_H

#define BTRFS_LOCKING_H

#include <linux/atomic.h>

#include <linux/wait.h>

#include <linux/percpu_counter.h>

#include "extent_io.h"

#define BTRFS_WRITE_LOCK 1

#define BTRFS_WRITE_LOCK_BLOCKING 3

#define BTRFS_READ_LOCK_BLOCKING 4

struct btrfs_path;

void btrfs_tree_lock(struct extent_buffer *eb);

void btrfs_tree_unlock(struct extent_buffer *eb);

		BUG();

}

struct btrfs_drew_lock {

	atomic_t readers;

	struct percpu_counter writers;

	wait_queue_head_t pending_writers;

	wait_queue_head_t pending_readers;

};

int btrfs_drew_lock_init(struct btrfs_drew_lock *lock);

void btrfs_drew_lock_destroy(struct btrfs_drew_lock *lock);

void btrfs_drew_write_lock(struct btrfs_drew_lock *lock);

bool btrfs_drew_try_write_lock(struct btrfs_drew_lock *lock);

void btrfs_drew_write_unlock(struct btrfs_drew_lock *lock);

void btrfs_drew_read_lock(struct btrfs_drew_lock *lock);

void btrfs_drew_read_unlock(struct btrfs_drew_lock *lock);

#endif