From 2992df73268f78ec9281692b9b44ae92f3933b54 Mon Sep 17 00:00:00 2001 From: Nikolay Borisov Date: Thu, 30 Jan 2020 14:59:44 +0200 Subject: btrfs: Implement DREW lock 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 Reviewed-by: David Sterba Signed-off-by: David Sterba --- fs/btrfs/locking.h | 20 ++++++++++++++++++++ 1 file changed, 20 insertions(+) (limited to 'fs/btrfs/locking.h') diff --git a/fs/btrfs/locking.h b/fs/btrfs/locking.h index 21a285883e89..d715846c10b8 100644 --- a/fs/btrfs/locking.h +++ b/fs/btrfs/locking.h @@ -6,6 +6,9 @@ #ifndef BTRFS_LOCKING_H #define BTRFS_LOCKING_H +#include +#include +#include #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 -- cgit v1.2.3-73-gaa49b