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btrfs: Implement DREW lock

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A (D)ouble (R)eader (W)riter (E)xclustion lock is a locking primitive
that allows to have multiple readers or multiple writers but not
multiple readers and writers holding it concurrently.

The code is factored out from the existing open-coded locking scheme
used to exclude pending snapshots from nocow writers and vice-versa.
Current implementation actually favors Readers (that is snapshot
creaters) to writers (nocow writers of the filesystem).

The API provides lock/unlock/trylock for reads and writes.

Formal specification for TLA+ provided by Valentin Schneider is at
https://lore.kernel.org/linux-btrfs/2dcaf81c-f0d3-409e-cb29-733d8b3b4cc9@arm.com/

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
Nikolay Borisov
2020-01-30 14:59:44 +02:00
committed by David Sterba
parent fd8efa818c
commit 2992df7326
3 changed files with 114 additions and 0 deletions
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20
fs/btrfs/locking.h
View File

@@ -6,6 +6,9 @@
#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
@@ -13,6 +16,8 @@
#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);
@@ -48,4 +53,19 @@ static inline void btrfs_tree_unlock_rw(struct extent_buffer *eb, int rw)
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