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Message-ID: <b9c33b04-08d7-87a1-3d93-d81a84e6af12@I-love.SAKURA.ne.jp>
Date: Wed, 16 May 2018 22:05:06 +0900
From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To: syzbot <syzbot+c4f9cebf9d651f6e54de@...kaller.appspotmail.com>,
linux-block@...r.kernel.org, syzkaller-bugs@...glegroups.com,
Dan Williams <dan.j.williams@...el.com>,
Jens Axboe <axboe@...nel.dk>
Cc: linux-kernel@...r.kernel.org, linux-ext4@...r.kernel.org,
dvyukov@...gle.com,
Alan Jenkins <alan.christopher.jenkins@...il.com>,
Bart Van Assche <bart.vanassche@....com>,
Christoph Hellwig <hch@....de>,
Hannes Reinecke <hare@...e.com>,
Johannes Thumshirn <jthumshirn@...e.de>,
Keith Busch <keith.busch@...el.com>,
"Martin K. Petersen" <martin.petersen@...cle.com>,
Martin Steigerwald <martin@...htvoll.de>,
Ming Lei <ming.lei@...hat.com>,
Oleksandr Natalenko <oleksandr@...alenko.name>,
Ross Zwisler <ross.zwisler@...ux.intel.com>
Subject: Re: INFO: task hung in blk_queue_enter
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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