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: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/discard.c b/fs/btrfs/discard.c
index 36431d7e1334cb3a04d9d26e9ec7c6dc63775ada..d641f451f8405b298380858790a4ff685b096660 100644 (file)
--- a/fs/btrfs/discard.c
+++ b/fs/btrfs/discard.c
@@ -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);