dm snapshot: avoid having two exceptions for the same chunk

We need to check if the exception was completed after dropping the lock.

After regaining the lock, __find_pending_exception checks if the exception
was already placed into &s->pending hash.

But we don't check if the exception was already completed and placed into
&s->complete hash. If the process waiting in alloc_pending_exception was
delayed at this point because of a scheduling latency and the exception
was meanwhile completed, we'd miss that and allocate another pending
exception for already completed chunk.

It would lead to a situation where two records for the same chunk exist
and potential data corruption because multiple snapshot I/Os to the
affected chunk could be redirected to different locations in the
snapshot.

Cc: stable@kernel.org
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

drivers/md/dm-snap.c

@@ -1080,6 +1080,13 @@ static int snapshot_map(struct dm_target *ti, struct bio *bio,
 				goto out_unlock;
 			}
 
+			e = lookup_exception(&s->complete, chunk);
+			if (e) {
+				free_pending_exception(pe);
+				remap_exception(s, e, bio, chunk);
+				goto out_unlock;
+			}
+
 			pe = __find_pending_exception(s, pe, chunk);
 			if (!pe) {
 				__invalidate_snapshot(s, -ENOMEM);
@@ -1226,6 +1233,12 @@ static int __origin_write(struct list_head *snapshots, struct bio *bio)
 				goto next_snapshot;
 			}
 
+			e = lookup_exception(&snap->complete, chunk);
+			if (e) {
+				free_pending_exception(pe);
+				goto next_snapshot;
+			}
+
 			pe = __find_pending_exception(snap, pe, chunk);
 			if (!pe) {
 				__invalidate_snapshot(snap, -ENOMEM);