On Sun, May 28, 2017 at 04:10:09PM +0000, Bart Van Assche wrote:

On Sun, 2017-05-28 at 18:44 +0800, Ming Lei wrote:

quoted

On Sat, May 27, 2017 at 09:46:45PM +0000, Bart Van Assche wrote:

quoted

On Sat, 2017-05-27 at 22:21 +0800, Ming Lei wrote:

quoted

 bool blk_mq_can_queue(struct blk_mq_hw_ctx *hctx)

@@ -1108,13 +1119,15 @@ static void __blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx)
 
 	if (!(hctx->flags & BLK_MQ_F_BLOCKING)) {
 		rcu_read_lock();
-		blk_mq_sched_dispatch_requests(hctx);
+		if (!blk_queue_quiesced(hctx->queue))
+			blk_mq_sched_dispatch_requests(hctx);
 		rcu_read_unlock();
 	} else {
 		might_sleep();
 
 		srcu_idx = srcu_read_lock(&hctx->queue_rq_srcu);
-		blk_mq_sched_dispatch_requests(hctx);
+		if (!blk_queue_quiesced(hctx->queue))
+			blk_mq_sched_dispatch_requests(hctx);
 		srcu_read_unlock(&hctx->queue_rq_srcu, srcu_idx);
 	}
 }

Sorry but I don't like these changes. Why have the blk_queue_quiesced()
calls be added at other code locations than the blk_mq_hctx_stopped() calls?
This will make the block layer unnecessary hard to maintain. Please consider
to change the blk_mq_hctx_stopped(hctx) calls in blk_mq_sched_dispatch_requests()
and *blk_mq_*run_hw_queue*() into blk_mq_hctx_stopped(hctx) || blk_queue_quiesced(q).

One benefit is that we make it explicit that the flag has to be checked
inside the RCU read-side critical sections. If you put it somewhere,
someone may put it out of read-side critical sections in future.

Hello Ming,

I really would like to see the blk_queue_quiesced() tests as close as possible to
the blk_mq_hctx_stopped() tests. But I agree that we need a way to document and/or

Could you explain why we have to do that? And checking on stopped state
doesn't need to hold RCU/SRCU read lock, and that two states are really
different.

verify that these tests occur with an RCU read-side lock held. Have you considered
to use rcu_read_lock_held() to document this?

Then we need to check if it is RCU or SRCU, and make code ugly as
current check on BLOCKING.

Thanks,
Ming