[PATCH] block: Fix a race in the runtime power management code

[Bart Van Assche <bvanassche@xxxxxxx>]

With the current implementation the following race can happen:
* blk_pre_runtime_suspend() calls blk_freeze_queue_start() and
  blk_mq_unfreeze_queue().
* blk_queue_enter() calls blk_queue_pm_only() and that function returns
  true.
* blk_queue_enter() calls blk_pm_request_resume() and that function does
  not call pm_request_resume() because the queue runtime status is
  RPM_ACTIVE.
* blk_pre_runtime_suspend() changes the queue status into RPM_SUSPENDING.

Fix this race by changing the queue runtime status into RPM_SUSPENDING
before switching q_usage_counter to atomic mode.

Cc: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
Cc: Stanley Chu <stanley.chu@xxxxxxxxxxxx>
Cc: Ming Lei <ming.lei@xxxxxxxxxx>
Cc: stable <stable@xxxxxxxxxxxxxxx>
Fixes: 986d413b7c15 ("blk-mq: Enable support for runtime power management")
Signed-off-by: Can Guo <cang@xxxxxxxxxxxxxx>
Signed-off-by: Bart Van Assche <bvanassche@xxxxxxx>
---
 block/blk-pm.c | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/block/blk-pm.c b/block/blk-pm.c
index b85234d758f7..17bd020268d4 100644
--- a/block/blk-pm.c
+++ b/block/blk-pm.c
@@ -67,6 +67,10 @@ int blk_pre_runtime_suspend(struct request_queue *q)
 
 	WARN_ON_ONCE(q->rpm_status != RPM_ACTIVE);
 
+	spin_lock_irq(&q->queue_lock);
+	q->rpm_status = RPM_SUSPENDING;
+	spin_unlock_irq(&q->queue_lock);
+
 	/*
 	 * Increase the pm_only counter before checking whether any
 	 * non-PM blk_queue_enter() calls are in progress to avoid that any
@@ -89,15 +93,14 @@ int blk_pre_runtime_suspend(struct request_queue *q)
 	/* Switch q_usage_counter back to per-cpu mode. */
 	blk_mq_unfreeze_queue(q);
 
-	spin_lock_irq(&q->queue_lock);
-	if (ret < 0)
+	if (ret < 0) {
+		spin_lock_irq(&q->queue_lock);
+		q->rpm_status = RPM_ACTIVE;
 		pm_runtime_mark_last_busy(q->dev);
-	else
-		q->rpm_status = RPM_SUSPENDING;
-	spin_unlock_irq(&q->queue_lock);
+		spin_unlock_irq(&q->queue_lock);
 
-	if (ret)
 		blk_clear_pm_only(q);
+	}
 
 	return ret;
 }