On Tue, Sep 10, 2024 at 07:22:16AM -0600, Jens Axboe wrote:

On 9/3/24 2:16 AM, Muchun Song wrote:

quoted

Supposing the following scenario.

CPU0                                        CPU1

blk_mq_insert_request()         1) store    blk_mq_unquiesce_queue()
blk_mq_run_hw_queue()                       blk_queue_flag_clear(QUEUE_FLAG_QUIESCED)       3) store
    if (blk_queue_quiesced())   2) load         blk_mq_run_hw_queues()
        return                                      blk_mq_run_hw_queue()
    blk_mq_sched_dispatch_requests()                    if (!blk_mq_hctx_has_pending())     4) load
                                                           return

The full memory barrier should be inserted between 1) and 2), as well as
between 3) and 4) to make sure that either CPU0 sees QUEUE_FLAG_QUIESCED is
cleared or CPU1 sees dispatch list or setting of bitmap of software queue.
Otherwise, either CPU will not re-run the hardware queue causing starvation.

So the first solution is to 1) add a pair of memory barrier to fix the
problem, another solution is to 2) use hctx->queue->queue_lock to synchronize
QUEUE_FLAG_QUIESCED. Here, we chose 2) to fix it since memory barrier is not
easy to be maintained.

Same comment here, 72-74 chars wide please.

quoted

diff --git a/block/blk-mq.c b/block/blk-mq.c
index b2d0f22de0c7f..ac39f2a346a52 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c

@@ -2202,6 +2202,24 @@ void blk_mq_delay_run_hw_queue(struct blk_mq_hw_ctx *hctx, unsigned long msecs)
 }
 EXPORT_SYMBOL(blk_mq_delay_run_hw_queue);
 
+static inline bool blk_mq_hw_queue_need_run(struct blk_mq_hw_ctx *hctx)
+{
+	bool need_run;
+
+	/*
+	 * When queue is quiesced, we may be switching io scheduler, or
+	 * updating nr_hw_queues, or other things, and we can't run queue
+	 * any more, even blk_mq_hctx_has_pending() can't be called safely.
+	 *
+	 * And queue will be rerun in blk_mq_unquiesce_queue() if it is
+	 * quiesced.
+	 */
+	__blk_mq_run_dispatch_ops(hctx->queue, false,
+				  need_run = !blk_queue_quiesced(hctx->queue) &&
+					      blk_mq_hctx_has_pending(hctx));
+	return need_run;
+}

This __blk_mq_run_dispatch_ops() is also way too wide, why didn't you
just break it like where you copied it from?

quoted

+
 /**
  * blk_mq_run_hw_queue - Start to run a hardware queue.
  * @hctx: Pointer to the hardware queue to run.

@@ -2222,20 +2240,23 @@ void blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx, bool async)
 
 	might_sleep_if(!async && hctx->flags & BLK_MQ_F_BLOCKING);
 
-	/*
-	 * When queue is quiesced, we may be switching io scheduler, or
-	 * updating nr_hw_queues, or other things, and we can't run queue
-	 * any more, even __blk_mq_hctx_has_pending() can't be called safely.
-	 *
-	 * And queue will be rerun in blk_mq_unquiesce_queue() if it is
-	 * quiesced.
-	 */
-	__blk_mq_run_dispatch_ops(hctx->queue, false,
-		need_run = !blk_queue_quiesced(hctx->queue) &&
-		blk_mq_hctx_has_pending(hctx));
+	need_run = blk_mq_hw_queue_need_run(hctx);
+	if (!need_run) {
+		unsigned long flags;
 
-	if (!need_run)
-		return;
+		/*
+		 * synchronize with blk_mq_unquiesce_queue(), becuase we check
+		 * if hw queue is quiesced locklessly above, we need the use
+		 * ->queue_lock to make sure we see the up-to-date status to
+		 * not miss rerunning the hw queue.
+		 */
+		spin_lock_irqsave(&hctx->queue->queue_lock, flags);
+		need_run = blk_mq_hw_queue_need_run(hctx);
+		spin_unlock_irqrestore(&hctx->queue->queue_lock, flags);
+
+		if (!need_run)
+			return;
+	}

Is this not solvable on the unquiesce side instead? It's rather a shame
to add overhead to the fast path to avoid a race with something that's
super unlikely, like quisce.

Yeah, it can be solved by adding synchronize_rcu()/srcu() in unquiesce
side, but SCSI may call it in non-sleepable context via scsi_internal_device_unblock_nowait().


Thanks,
Ming