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: d06107a
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>

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;
             }
         }

tests/qemu-iotests/151

@@ -143,10 +143,6 @@ class TestActiveMirror(iotests.QMPTestCase):
         self.potential_writes_in_flight = False
 
     def testIntersectingActiveIO(self):
-        # FIXME: test-case is dead-locking. To reproduce dead-lock just drop
-        # this return statement
-        return
-
         # Fill the source image
         result = self.vm.hmp_qemu_io('source', 'write -P 1 0 2M')
 
@@ -180,18 +176,14 @@ class TestActiveMirror(iotests.QMPTestCase):
 
         # Now we resumed 1, so 2 and 3 goes to the next iteration of while loop
         # in mirror_wait_on_conflicts(). They don't exit, as bitmap is dirty
-        # due to request 4. And they start to wait: 2 wait for 3, 3 wait for 2
-        # - DEAD LOCK.
-        # Note that it's important that we add request 4 at last: requests are
-        # appended to the list, so we are sure that 4 is last in the list, so 2
-        # and 3 now waits for each other, not for 4.
+        # due to request 4.
+        # In the past at that point 2 and 3 would wait for each other producing
+        # a dead-lock. Now this is fixed and they will wait for request 4.
 
         self.vm.hmp_qemu_io('source', 'resume B')
 
-        # Resuming 4 doesn't help, 2 and 3 already dead-locked
-        # To check the dead-lock run:
-        #    gdb -p $(pidof qemu-system-x86_64) -ex 'set $job=(MirrorBlockJob *)jobs.lh_first' -ex 'p *$job->ops_in_flight.tqh_first' -ex 'p *$job->ops_in_flight.tqh_first->next.tqe_next'
-        # You'll see two MirrorOp objects waiting on each other
+        # After resuming 4, one of 2 and 3 goes first and set in_flight_bitmap,
+        # so the other will wait for it.
 
         result = self.vm.qmp('block-job-set-speed', device='mirror', speed=0)
         self.assert_qmp(result, 'return', {})