nbd: make sure request completion won't concurrent

commit cddce01 ("nbd: Aovid double completion of a request")
try to fix that nbd_clear_que() and recv_work() can complete a
request concurrently. However, the problem still exists:

t1                    t2                     t3

nbd_disconnect_and_put
 flush_workqueue
                      recv_work
                       blk_mq_complete_request
                        blk_mq_complete_request_remote -> this is true
                         WRITE_ONCE(rq->state, MQ_RQ_COMPLETE)
                          blk_mq_raise_softirq
                                             blk_done_softirq
                                              blk_complete_reqs
                                               nbd_complete_rq
                                                blk_mq_end_request
                                                 blk_mq_free_request
                                                  WRITE_ONCE(rq->state, MQ_RQ_IDLE)
  nbd_clear_que
   blk_mq_tagset_busy_iter
    nbd_clear_req
                                                   __blk_mq_free_request
                                                    blk_mq_put_tag
     blk_mq_complete_request -> complete again

There are three places where request can be completed in nbd:
recv_work(), nbd_clear_que() and nbd_xmit_timeout(). Since they
all hold cmd->lock before completing the request, it's easy to
avoid the problem by setting and checking a cmd flag.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Link: https://lore.kernel.org/r/20210916093350.1410403-3-yukuai3@huawei.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/nbd.c

@@ -411,7 +411,11 @@ static enum blk_eh_timer_return nbd_xmit_timeout(struct request *req,
 	if (!mutex_trylock(&cmd->lock))
 		return BLK_EH_RESET_TIMER;
 
-	__clear_bit(NBD_CMD_INFLIGHT, &cmd->flags);
+	if (!__test_and_clear_bit(NBD_CMD_INFLIGHT, &cmd->flags)) {
+		mutex_unlock(&cmd->lock);
+		return BLK_EH_DONE;
+	}
+
 	if (!refcount_inc_not_zero(&nbd->config_refs)) {
 		cmd->status = BLK_STS_TIMEOUT;
 		mutex_unlock(&cmd->lock);
@@ -846,7 +850,10 @@ static bool nbd_clear_req(struct request *req, void *data, bool reserved)
 		return true;
 
 	mutex_lock(&cmd->lock);
-	__clear_bit(NBD_CMD_INFLIGHT, &cmd->flags);
+	if (!__test_and_clear_bit(NBD_CMD_INFLIGHT, &cmd->flags)) {
+		mutex_unlock(&cmd->lock);
+		return true;
+	}
 	cmd->status = BLK_STS_IOERR;
 	mutex_unlock(&cmd->lock);