Commit 28967c76 authored by Filipe Manana's avatar Filipe Manana Committed by David Sterba
Browse files

btrfs: remove redundant memory barrier from extent_io_tree_release() 



The memory barrier at extent_io_tree_release() is redundant. Holding
spin_lock here is not enough to drop the barrier completely.  We only
change the waitqueue of an extent state record while holding the tree
lock - see wait_on_state().

The update to waitqueue state will not become stale because there will
be an spin_unlock/spin_lock sequence between the change and waiting,
this implies a full memory barrier.

So remove the explicit smp_mb() barrier.

Signed-off-by: default avatarFilipe Manana <fdmanana@suse.com>
Reviewed-by: default avatarDavid Sterba <dsterba@suse.com>
[ reword reasoning ]
Signed-off-by: default avatarDavid Sterba <dsterba@suse.com>
parent a1c20d15

fs/btrfs/extent-io-tree.c

+5 −6
Original line number Diff line number Diff line
@@ -115,12 +115,6 @@ void extent_io_tree_init(struct btrfs_fs_info *fs_info, 
void extent_io_tree_release(struct extent_io_tree *tree)

{

	spin_lock(&tree->lock);

	/*

	 * Do a single barrier for the waitqueue_active check here, the state

	 * of the waitqueue should not change once extent_io_tree_release is

	 * called.

	 */

	smp_mb();

	while (!RB_EMPTY_ROOT(&tree->state)) {

		struct rb_node *node;

		struct extent_state *state;
@@ -130,6 +124,11 @@ void extent_io_tree_release(struct extent_io_tree *tree) 
		rb_erase(&state->rb_node, &tree->state);

		RB_CLEAR_NODE(&state->rb_node);

		ASSERT(!(state->state & EXTENT_LOCKED));

		/*

		 * No need for a memory barrier here, as we are holding the tree

		 * lock and we only change the waitqueue while holding that lock

		 * (see wait_on_state()).

		 */

		ASSERT(!waitqueue_active(&state->wq));

		free_extent_state(state);