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Subject[PATCH] blk-mq: fix hang issue in blk_queue_enter()
Date
There is a race between blk_queue_enter() and block layer's runtime
suspend.

CPU0 CPU1
--------------------------------- -------------------------------
blk_pre_runtime_suspend(q) { blk_queue_enter() {
/* q->rpm_status=RPM_ACTIVE */
blk_set_pm_only(q)
/* q->pm_only=1 */
blk_freeze_queue_start(q)
blk_mq_unfreeze_queue(q)
if (percpu_ref_tryget_live()) {
/* pm=0 && q->pm_only=1 */
if (pm || !blk_queue_pm_only(q)) {
} else {
percpu_ref_put()
}
}
wait_event(q->mq_freeze_wq,
(!q->mq_freeze_depth &&
/* q->rpm_status=RPM_ACTIVE
q->pm_only=1 */
(pm || (blk_pm_request_resume(q),
!blk_queue_pm_only(q)))) ||
blk_queue_dying(q))
}
spin_lock_irq(&q->queue_lock)
q->rpm_status = RPM_SUSPENDING
spin_unlock_irq(&q->queue_lock)
}

At this point blk_pm_request_resume() missed the chance to resume the
queue, so blk_queue_enter() may wait here forever.
The solution is to wake up the mq_freeze_wq after runtime suspend
completed, make blk_pm_request_resume() reexamine the q->rpm_status flag.

Signed-off-by: Yang Yang <yang.yang@vivo.com>
---
block/blk-pm.c | 2 ++
1 file changed, 2 insertions(+)

diff --git a/block/blk-pm.c b/block/blk-pm.c
index b85234d758f7..dec7d0aef606 100644
--- a/block/blk-pm.c
+++ b/block/blk-pm.c
@@ -132,6 +132,8 @@ void blk_post_runtime_suspend(struct request_queue *q, int err)

if (err)
blk_clear_pm_only(q);
+ else
+ wake_up_all(&q->mq_freeze_wq);
}
EXPORT_SYMBOL(blk_post_runtime_suspend);

--
2.17.1
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