block/mirror: fix active mirror dead-lock in mirror_wait_on_conflicts

It's possible that requests start to wait each other in
mirror_wait_on_conflicts(). To avoid it let's use same technique as in
block/io.c in bdrv_wait_serialising_requests_locked() /
bdrv_find_conflicting_request(): don't wait on intersecting request if
it is already waiting for some other request.

For details of the dead-lock look at testIntersectingActiveIO()
test-case which we actually fixing now.

Fixes: d06107ade0ce74dc39739bac80de84b51ec18546
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20210702211636.228981-4-vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 12 insertions, 0 deletions

diff --git a/block/mirror.c b/block/mirror.c
index ad6aac2..98fc66e 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -107,6 +107,7 @@ struct MirrorOp {
     bool is_in_flight;
     CoQueue waiting_requests;
     Coroutine *co;
+    MirrorOp *waiting_for_op;
 
     QTAILQ_ENTRY(MirrorOp) next;
 };
@@ -159,7 +160,18 @@ static void coroutine_fn mirror_wait_on_conflicts(MirrorOp *self,
             if (ranges_overlap(self_start_chunk, self_nb_chunks,
                                op_start_chunk, op_nb_chunks))
             {
+                /*
+                 * If the operation is already (indirectly) waiting for us, or
+                 * will wait for us as soon as it wakes up, then just go on
+                 * (instead of producing a deadlock in the former case).
+                 */
+                if (op->waiting_for_op) {
+                    continue;
+                }
+
+                self->waiting_for_op = op;
                 qemu_co_queue_wait(&op->waiting_requests, NULL);
+                self->waiting_for_op = NULL;
                 break;
             }
         }