btrfs: convert snapshot/nocow exlcusion to drew lock

This patch removes all haphazard code implementing nocow writers
exclusion from pending snapshot creation and switches to using the drew
lock to ensure this invariant still holds.

'Readers' are snapshot creators from create_snapshot and 'writers' are
nocow writers from buffered write path or btrfs_setsize. This locking
scheme allows for multiple snapshots to happen while any nocow writers
are blocked, since writes to page cache in the nocow path will make
snapshots inconsistent.

So for performance reasons we'd like to have the ability to run multiple
concurrent snapshots and also favors readers in this case. And in case
there aren't pending snapshots (which will be the majority of the cases)
we rely on the percpu's writers counter to avoid cacheline contention.

The main gain from using the drew lock is it's now a lot easier to
reason about the guarantees of the locking scheme and whether there is
some silent breakage lurking.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 3 insertions, 7 deletions

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index ab8151247b93..db9e872bcc79 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -957,11 +957,6 @@ static inline struct btrfs_fs_info *btrfs_sb(struct super_block *sb)
 	return sb->s_fs_info;
 }
 
-struct btrfs_subvolume_writers {
-	struct percpu_counter	counter;
-	wait_queue_head_t	wait;
-};
-
 /*
  * The state of btrfs root
  */
@@ -1133,8 +1128,9 @@ struct btrfs_root {
 	 * root_item_lock.
 	 */
 	int dedupe_in_progress;
-	struct btrfs_subvolume_writers *subv_writers;
-	atomic_t will_be_snapshotted;
+	/* For exclusion of snapshot creation and nocow writes */
+	struct btrfs_drew_lock snapshot_lock;
+
 	atomic_t snapshot_force_cow;
 
 	/* For qgroup metadata reserved space */