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From: Dmitry Vyukov <dvyukov@google.com>
To: Bart Van Assche <Bart.VanAssche@wdc.com>
Cc: "syzbot+c4f9cebf9d651f6e54de@syzkaller.appspotmail.com"
	<syzbot+c4f9cebf9d651f6e54de@syzkaller.appspotmail.com>,
	"syzkaller-bugs@googlegroups.com"
	<syzkaller-bugs@googlegroups.com>,
	"dan.j.williams@intel.com" <dan.j.williams@intel.com>,
	"linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
	"penguin-kernel@I-love.SAKURA.ne.jp"
	<penguin-kernel@i-love.sakura.ne.jp>,
	"axboe@kernel.dk" <axboe@kernel.dk>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"jthumshirn@suse.de" <jthumshirn@suse.de>,
	"alan.christopher.jenkins@gmail.com"
	<alan.christopher.jenkins@gmail.com>, "hch@lst.de" <hch@lst.de>,
	"martin.petersen@oracle.com" <martin.petersen@oracle.com>,
	"ming.lei@redhat.com" <ming.lei@redhat.com>,
	"martin@lichtvoll.de" <martin@lichtvoll.de>,
	"oleksandr@natalenko.name" <oleksandr@natalenko.name>,
	"hare@suse.com" <hare@suse.com>,
	"ross.zwisler@linux.intel.com" <ross.zwisler@linux.intel.com>,
	"keith.busch@intel.com" <keith.busch@intel.com>,
	"linux-ext4@vger.kernel.org" <linux-ext4@vger.kernel.org>
Subject: Re: INFO: task hung in blk_queue_enter
Date: Wed, 16 May 2018 17:16:40 +0200	[thread overview]
Message-ID: <CACT4Y+ZJwvROpBmKWF08fTd2AH=dO-a5+dzCzZua_vVFjiCRBw@mail.gmail.com> (raw)
In-Reply-To: <43327033306c3dd2f7c3717d64ce22415b6f3451.camel@wdc.com>

On Wed, May 16, 2018 at 4:56 PM, Bart Van Assche <Bart.VanAssche@wdc.com> wrote:
> On Wed, 2018-05-16 at 22:05 +0900, Tetsuo Handa wrote:
>> 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;
>>       }
>>  }
>
> That change looks wrong to me.

Hi Bart,

Why does it look wrong to you?

> Additionally, I think that you are looking in
> the wrong direction. Since blk_mq_freeze_queue_wait() and blk_queue_enter()
> work fine for all block drivers except the loop driver I think that you should
> have a closer look at how the loop driver uses this block layer functionality.
>
> Thanks,
>
> Bart.
>
>
>

WARNING: multiple messages have this Message-ID
From: Dmitry Vyukov <dvyukov@google.com>
To: Bart Van Assche <Bart.VanAssche@wdc.com>
Cc: "syzbot+c4f9cebf9d651f6e54de@syzkaller.appspotmail.com"
	<syzbot+c4f9cebf9d651f6e54de@syzkaller.appspotmail.com>,
	"syzkaller-bugs@googlegroups.com"
	<syzkaller-bugs@googlegroups.com>,
	"dan.j.williams@intel.com" <dan.j.williams@intel.com>,
	"linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
	"penguin-kernel@I-love.SAKURA.ne.jp"
	<penguin-kernel@i-love.sakura.ne.jp>,
	"axboe@kernel.dk" <axboe@kernel.dk>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"jthumshirn@suse.de" <jthumshirn@suse.de>,
	"alan.christopher.jenkins@gmail.com"
	<alan.christopher.jenkins@gmail.com>, "hch@lst.de" <hch@lst.de>,
	"martin.petersen@oracle.com" <martin.petersen@oracle.com>,
	"ming.lei@redhat.com" <ming.lei@redhat.com>,
	"martin@lichtvoll.de" <martin@lichtvoll.de>,
	"oleks
Subject: Re: INFO: task hung in blk_queue_enter
Date: Wed, 16 May 2018 17:16:40 +0200	[thread overview]
Message-ID: <CACT4Y+ZJwvROpBmKWF08fTd2AH=dO-a5+dzCzZua_vVFjiCRBw@mail.gmail.com> (raw)
In-Reply-To: <43327033306c3dd2f7c3717d64ce22415b6f3451.camel@wdc.com>

On Wed, May 16, 2018 at 4:56 PM, Bart Van Assche <Bart.VanAssche@wdc.com> wrote:
> On Wed, 2018-05-16 at 22:05 +0900, Tetsuo Handa wrote:
>> 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;
>>       }
>>  }
>
> That change looks wrong to me.

Hi Bart,

Why does it look wrong to you?

> Additionally, I think that you are looking in
> the wrong direction. Since blk_mq_freeze_queue_wait() and blk_queue_enter()
> work fine for all block drivers except the loop driver I think that you should
> have a closer look at how the loop driver uses this block layer functionality.
>
> Thanks,
>
> Bart.
>
>
>

  reply	other threads:[~2018-05-16 15:16 UTC|newest]

Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-04-28 11:24 syzbot
2018-05-15 11:45 ` Tetsuo Handa
2018-05-16 13:05   ` Tetsuo Handa
2018-05-16 14:56     ` Bart Van Assche
2018-05-16 14:56       ` Bart Van Assche
2018-05-16 15:16       ` Dmitry Vyukov [this message]
2018-05-16 15:16         ` Dmitry Vyukov
2018-05-16 15:37         ` Bart Van Assche
2018-05-16 15:37           ` Bart Van Assche
2018-05-16 15:37           ` Bart Van Assche
2018-05-21 21:52           ` Tetsuo Handa
2018-05-22 11:20             ` Tetsuo Handa
2018-06-01 10:10               ` Tetsuo Handa
2018-06-01 17:52                 ` Jens Axboe
2018-06-01 23:49                   ` Ming Lei
2018-06-02  0:49                     ` Jens Axboe
2018-06-02  0:56                       ` Jens Axboe
2018-06-02  2:36                       ` Ming Lei
2018-06-02  4:31                         ` Jens Axboe
2018-06-02  4:54                           ` Ming Lei
2018-06-02  8:07                             ` Martin Steigerwald
2018-06-02  8:07                               ` Martin Steigerwald
2018-06-02 13:48                             ` Jens Axboe
2018-06-02 13:48                               ` Jens Axboe
2018-06-05  0:27                   ` Tetsuo Handa
2018-06-05  0:41                     ` Ming Lei
2018-06-07  3:29                       ` Ming Lei
2018-06-07  3:29                         ` Ming Lei
2018-06-07 13:19                         ` Tetsuo Handa
2018-06-04 11:46                 ` Dmitry Vyukov
2018-06-04 13:13                   ` Tetsuo Handa
2018-05-16 17:33     ` Alan Jenkins

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