blk_mq: call preempt_disable/enable in blk_mq_run_hw_queue, and only if needed

preempt_disable/enable surrounds every call to blk_mq_run_hw_queue,
except the one in blk-flush.c.  In fact that one is always asynchronous,
and it does not need smp_processor_id().

We can do the same for all other calls, avoiding preempt_disable when
async is true.  This avoids peppering blk-mq.c with preemption-disabled
regions.

Cc: Jens Axboe <axboe@kernel.dk>
Cc: Thomas Gleixner <tglx@linutronix.de>
Reported-by: NClark Williams <williams@redhat.com>
Tested-by: NClark Williams <williams@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NJens Axboe <axboe@fb.com>

block/blk-mq.c

@@ -801,9 +801,18 @@ void blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx, bool async)
 	if (unlikely(test_bit(BLK_MQ_S_STOPPED, &hctx->state)))
 		return;
 
-	if (!async && cpumask_test_cpu(smp_processor_id(), hctx->cpumask))
+	if (!async) {
+		preempt_disable();
+		if (cpumask_test_cpu(smp_processor_id(), hctx->cpumask)) {
 			__blk_mq_run_hw_queue(hctx);
-	else if (hctx->queue->nr_hw_queues == 1)
+			preempt_enable();
+			return;
+		}
+
+		preempt_enable();
+	}
+
+	if (hctx->queue->nr_hw_queues == 1)
 		kblockd_schedule_delayed_work(&hctx->run_work, 0);
 	else {
 		unsigned int cpu;
@@ -824,9 +833,7 @@ void blk_mq_run_queues(struct request_queue *q, bool async)
 		    test_bit(BLK_MQ_S_STOPPED, &hctx->state))
 			continue;
 
-		preempt_disable();
 		blk_mq_run_hw_queue(hctx, async);
-		preempt_enable();
 	}
 }
 EXPORT_SYMBOL(blk_mq_run_queues);
@@ -853,9 +860,7 @@ void blk_mq_start_hw_queue(struct blk_mq_hw_ctx *hctx)
 {
 	clear_bit(BLK_MQ_S_STOPPED, &hctx->state);
 
-	preempt_disable();
 	blk_mq_run_hw_queue(hctx, false);
-	preempt_enable();
 }
 EXPORT_SYMBOL(blk_mq_start_hw_queue);
 
@@ -880,9 +885,7 @@ void blk_mq_start_stopped_hw_queues(struct request_queue *q, bool async)
 			continue;
 
 		clear_bit(BLK_MQ_S_STOPPED, &hctx->state);
-		preempt_disable();
 		blk_mq_run_hw_queue(hctx, async);
-		preempt_enable();
 	}
 }
 EXPORT_SYMBOL(blk_mq_start_stopped_hw_queues);