null_blk: guarantee device restart in all irq modes

In single-queue (block layer) mode,the function null_rq_prep_fn stops
the device if alloc_cmd fails. Then, once stopped, the device must be
restarted on the next command completion, so that the request(s) for
which alloc_cmd failed can be requeued. Otherwise the device hangs.

Unfortunately, device restart is currently performed only for delayed
completions, i.e., in irqmode==2. This fact causes hangs, for the
above reasons, with the other irqmodes in combination with single-queue
block layer.

This commits addresses this issue by making sure that, if stopped, the
device is properly restarted for all irqmodes on completions.
Signed-off-by: Paolo Valente <paolo.valente@unimore.it>
Signed-off-by: Arianna AVanzini <avanzini@google.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

drivers/block/null_blk.c

@@ -217,6 +217,8 @@ static struct nullb_cmd *alloc_cmd(struct nullb_queue *nq, int can_wait)
 
 static void end_cmd(struct nullb_cmd *cmd)
 {
+	struct request_queue *q = NULL;
+
 	switch (queue_mode)  {
 	case NULL_Q_MQ:
 		blk_mq_end_request(cmd->rq, 0);
@@ -227,27 +229,28 @@ static void end_cmd(struct nullb_cmd *cmd)
 		break;
 	case NULL_Q_BIO:
 		bio_endio(cmd->bio);
-		break;
+		goto free_cmd;
 	}
 
-	free_cmd(cmd);
-}
-
-static enum hrtimer_restart null_cmd_timer_expired(struct hrtimer *timer)
-{
-	struct nullb_cmd *cmd = container_of(timer, struct nullb_cmd, timer);
-	struct request_queue *q = NULL;
-
 	if (cmd->rq)
 		q = cmd->rq->q;
 
+	/* Restart queue if needed, as we are freeing a tag */
 	if (q && !q->mq_ops && blk_queue_stopped(q)) {
-		spin_lock(q->queue_lock);
+		unsigned long flags;
+
+		spin_lock_irqsave(q->queue_lock, flags);
 		if (blk_queue_stopped(q))
 			blk_start_queue(q);
-		spin_unlock(q->queue_lock);
+		spin_unlock_irqrestore(q->queue_lock, flags);
 	}
-	end_cmd(cmd);
+free_cmd:
+	free_cmd(cmd);
+}
+
+static enum hrtimer_restart null_cmd_timer_expired(struct hrtimer *timer)
+{
+	end_cmd(container_of(timer, struct nullb_cmd, timer));
 
 	return HRTIMER_NORESTART;
 }