[PATCH v2 2/3] btrfs: fix racy access to discard_ctl data

[Pavel Begunkov <asml.silence@gmail.com>]

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

That may lead 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: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
 fs/btrfs/discard.c | 10 +++-------
 1 file changed, 3 insertions(+), 7 deletions(-)

diff --git a/fs/btrfs/discard.c b/fs/btrfs/discard.c
index 4ca852445873..a338f44fec3b 100644
--- a/fs/btrfs/discard.c
+++ b/fs/btrfs/discard.c
@@ -475,13 +475,6 @@ 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) {
@@ -497,7 +490,10 @@ 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);
 
-- 
2.24.0