btrfs: get zone unusable bytes while holding lock at btrfs_reclaim_bgs_work()

At btrfs_reclaim_bgs_work(), we are grabbing a block group's zone unusable
bytes while not under the protection of the block group's spinlock, so
this can trigger race reports from KCSAN (or similar tools) since that
field is typically updated while holding the lock, such as at
__btrfs_add_free_space_zoned() for example.

Fix this by grabbing the zone unusable bytes while we are still in the
critical section holding the block group's spinlock, which is right above
where we are currently grabbing it.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/block-group.c b/fs/btrfs/block-group.c
index 18f58674a16c2dc7d76ba2ee8c97798dce7f546c..cbea2bf257143c83fb38f6b81e980c119ac43772 100644 (file)
--- a/fs/btrfs/block-group.c
+++ b/fs/btrfs/block-group.c
@@ -1887,6 +1887,17 @@ void btrfs_reclaim_bgs_work(struct work_struct *work)
                        up_write(&space_info->groups_sem);
                        goto next;
                }
+
+               /*
+                * Cache the zone_unusable value before turning the block group
+                * to read only. As soon as the block group is read only it's
+                * zone_unusable value gets moved to the block group's read-only
+                * bytes and isn't available for calculations anymore. We also
+                * cache it before unlocking the block group, to prevent races
+                * (reports from KCSAN and such tools) with tasks updating it.
+                */
+               zone_unusable = bg->zone_unusable;
+
                spin_unlock(&bg->lock);
                spin_unlock(&space_info->lock);
 
@@ -1903,13 +1914,6 @@ void btrfs_reclaim_bgs_work(struct work_struct *work)
                        goto next;
                }
 
-               /*
-                * Cache the zone_unusable value before turning the block group
-                * to read only. As soon as the blog group is read only it's
-                * zone_unusable value gets moved to the block group's read-only
-                * bytes and isn't available for calculations anymore.
-                */
-               zone_unusable = bg->zone_unusable;
                ret = inc_block_group_ro(bg, 0);
                up_write(&space_info->groups_sem);
                if (ret < 0)