Commit 1ea2872f authored by Pavel Begunkov's avatar Pavel Begunkov Committed by David Sterba
Browse files

btrfs: fix racy access to discard_ctl data 



Because only one discard worker may be running at any given point, it
could have been safe to modify ->prev_discard, etc. without
synchronization, if not for @override flag in
btrfs_discard_schedule_work() and delayed_work_pending() returning false
while workfn is running.

That may lead to torn reads of u64 for some architectures, but that's
not a big problem as only slightly affects the discard rate.

Suggested-by: default avatarJosef Bacik <josef@toxicpanda.com>
Reviewed-by: default avatarJosef Bacik <josef@toxicpanda.com>
Signed-off-by: default avatarPavel Begunkov <asml.silence@gmail.com>
Reviewed-by: default avatarDavid Sterba <dsterba@suse.com>
Signed-off-by: default avatarDavid Sterba <dsterba@suse.com>
parent ea9ed87c

fs/btrfs/discard.c

+3 −7
Original line number Diff line number Diff line
@@ -477,13 +477,6 @@ static void btrfs_discard_workfn(struct work_struct *work) 
		discard_ctl->discard_extent_bytes += trimmed;

	}



	/*

	 * Updated without locks as this is inside the workfn and nothing else

	 * is reading the values

	 */

	discard_ctl->prev_discard = trimmed;

	discard_ctl->prev_discard_time = ktime_get_ns();



	/* Determine next steps for a block_group */

	if (block_group->discard_cursor >= btrfs_block_group_end(block_group)) {

		if (discard_state == BTRFS_DISCARD_BITMAPS) {
@@ -499,7 +492,10 @@ static void btrfs_discard_workfn(struct work_struct *work) 
		}

	}



	now = ktime_get_ns();

	spin_lock(&discard_ctl->lock);

	discard_ctl->prev_discard = trimmed;

	discard_ctl->prev_discard_time = now;

	discard_ctl->block_group = NULL;

	spin_unlock(&discard_ctl->lock);