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SubjectRe: INFO: task hung in blk_queue_enter
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Tetsuo Handa wrote:
> I couldn't check whether freeze_depth in blk_freeze_queue_start() was 1,
> but presumably q->mq_freeze_depth > 0 because syz-executor7(PID=5010) is
> stuck at wait_event() in blk_queue_enter().
>
> Since flags == 0, preempt == false. Since stuck at wait_event(), success == false.
> Thus, atomic_read(&q->mq_freeze_depth) > 0 if blk_queue_dying(q) == false. And I
> guess blk_queue_dying(q) == false because we are just trying to freeze/unfreeze.
>

I was able to reproduce the hung up using modified reproducer, and got values
using below debug printk() patch.

--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -950,10 +950,12 @@ int blk_queue_enter(struct request_queue *q, blk_mq_req_flags_t flags)
*/
smp_rmb();

- wait_event(q->mq_freeze_wq,
- (atomic_read(&q->mq_freeze_depth) == 0 &&
- (preempt || !blk_queue_preempt_only(q))) ||
- blk_queue_dying(q));
+ while (wait_event_timeout(q->mq_freeze_wq,
+ (atomic_read(&q->mq_freeze_depth) == 0 &&
+ (preempt || !blk_queue_preempt_only(q))) ||
+ blk_queue_dying(q), 10 * HZ) == 0)
+ printk("%s(%u): q->mq_freeze_depth=%d preempt=%d blk_queue_preempt_only(q)=%d blk_queue_dying(q)=%d\n",
+ current->comm, current->pid, atomic_read(&q->mq_freeze_depth), preempt, blk_queue_preempt_only(q), blk_queue_dying(q));
if (blk_queue_dying(q))
return -ENODEV;
}

[ 75.869126] print_req_error: I/O error, dev loop0, sector 0
[ 85.983146] a.out(8838): q->mq_freeze_depth=1 preempt=0 blk_queue_preempt_only(q)=0 blk_queue_dying(q)=0
[ 96.222884] a.out(8838): q->mq_freeze_depth=1 preempt=0 blk_queue_preempt_only(q)=0 blk_queue_dying(q)=0
[ 106.463322] a.out(8838): q->mq_freeze_depth=1 preempt=0 blk_queue_preempt_only(q)=0 blk_queue_dying(q)=0
[ 116.702912] a.out(8838): q->mq_freeze_depth=1 preempt=0 blk_queue_preempt_only(q)=0 blk_queue_dying(q)=0

One ore more threads are waiting for q->mq_freeze_depth to become 0. But the
thread who incremented q->mq_freeze_depth at blk_freeze_queue_start(q) from
blk_freeze_queue() is waiting at blk_mq_freeze_queue_wait(). Therefore,
atomic_read(&q->mq_freeze_depth) == 0 condition for wait_event() in
blk_queue_enter() will never be satisfied. But what does that wait_event()
want to do? Isn't "start freezing" a sort of blk_queue_dying(q) == true?
Since percpu_ref_tryget_live(&q->q_usage_counter) failed and the queue is
about to be frozen, shouldn't we treat atomic_read(&q->mq_freeze_depth) != 0
as if blk_queue_dying(q) == true? That is, something like below:

diff --git a/block/blk-core.c b/block/blk-core.c
index 85909b4..59e2496 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -951,10 +951,10 @@ int blk_queue_enter(struct request_queue *q, blk_mq_req_flags_t flags)
smp_rmb();

wait_event(q->mq_freeze_wq,
- (atomic_read(&q->mq_freeze_depth) == 0 &&
- (preempt || !blk_queue_preempt_only(q))) ||
+ atomic_read(&q->mq_freeze_depth) ||
+ (preempt || !blk_queue_preempt_only(q)) ||
blk_queue_dying(q));
- if (blk_queue_dying(q))
+ if (atomic_read(&q->mq_freeze_depth) || blk_queue_dying(q))
return -ENODEV;
}
}
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