dm cache: do not add migration to completed list before unhooking bio

When completing an overwrite bio, in overwrite_endio(), the associated
migration should not be added to the 'completed_migrations' until the
bio's fields are restored with dm_unhook_bio().

Otherwise, do_worker() can race to process 'completed_migrations' before
dm_unhook_bio() -- so the bio's bi_end_io is incorrect.  This is
unlikely to cause any problems given the current code but should be
fixed on the basis of correctness.

Also, the cache's spinlock only needs to be held when manipulating the
'completed_migrations' list -- other changes don't need protection.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Acked-by: Joe Thornber <ejt@redhat.com>

1 files changed, 4 insertions, 2 deletions

diff --git a/drivers/md/dm-cache-target.c b/drivers/md/dm-cache-target.c
index 14256b7fce79..db0944465127 100644
--- a/drivers/md/dm-cache-target.c
+++ b/drivers/md/dm-cache-target.c
@@ -1010,13 +1010,15 @@ static void overwrite_endio(struct bio *bio, int err)
 	struct per_bio_data *pb = get_per_bio_data(bio, pb_data_size);
 	unsigned long flags;
 
+	dm_unhook_bio(&pb->hook_info, bio);
+
 	if (err)
 		mg->err = true;
 
+	mg->requeue_holder = false;
+
 	spin_lock_irqsave(&cache->lock, flags);
 	list_add_tail(&mg->list, &cache->completed_migrations);
-	dm_unhook_bio(&pb->hook_info, bio);
-	mg->requeue_holder = false;
 	spin_unlock_irqrestore(&cache->lock, flags);
 
 	wake_worker(cache);