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Message-Id: <201806050027.w550RfJl010157@www262.sakura.ne.jp>
Subject: Re: INFO: task hung in =?ISO-2022-JP?B?YmxrX3F1ZXVlX2VudGVy?=
From:   Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
To:     Jens Axboe <axboe@kernel.dk>
Cc:     Bart.VanAssche@wdc.com, dvyukov@google.com,
        linux-kernel@vger.kernel.org, linux-block@vger.kernel.org,
        jthumshirn@suse.de, alan.christopher.jenkins@gmail.com,
        syzbot+c4f9cebf9d651f6e54de@syzkaller.appspotmail.com,
        martin.petersen@oracle.com, dan.j.williams@intel.com, hch@lst.de,
        oleksandr@natalenko.name, ming.lei@redhat.com, martin@lichtvoll.de,
        hare@suse.com, syzkaller-bugs@googlegroups.com,
        ross.zwisler@linux.intel.com, keith.busch@intel.com,
        linux-ext4@vger.kernel.org
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Date:   Tue, 05 Jun 2018 09:27:41 +0900
References: <25708e84-6f35-04c3-a2e4-6854f0ed9e78@I-love.SAKURA.ne.jp> <dc9862c7-ec16-8b11-04e4-422b435d41ef@kernel.dk>
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Jens Axboe wrote:
> On 6/1/18 4:10 AM, Tetsuo Handa wrote:
> > Tetsuo Handa wrote:
> >> Since sum of percpu_count did not change after percpu_ref_kill(), this is
> >> not a race condition while folding percpu counter values into atomic counter
> >> value. That is, for some reason, someone who is responsible for calling
> >> percpu_ref_put(&q->q_usage_counter) (presumably via blk_queue_exit()) is
> >> unable to call percpu_ref_put().
> >> But I don't know how to find someone who is failing to call percpu_ref_put()...
> > 
> > I found the someone. It was already there in the backtrace...
> > 
> 
> Ahh, nicely spotted! One idea would be the one below. For this case,
> we're recursing, so we can either do a non-block queue enter, or we
> can just do a live enter.
> 

While "block: don't use blocking queue entered for recursive bio submits" was
already applied, syzbot is still reporting a hung task with same signature but
different trace.

https://syzkaller.appspot.com/text?tag=CrashLog&x=1432cedf800000
----------------------------------------
[  492.512243] INFO: task syz-executor1:20263 blocked for more than 120 seconds.
[  492.519604]       Not tainted 4.17.0+ #83
[  492.523793] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[  492.531787] syz-executor1   D23384 20263   4574 0x00000004
[  492.537443] Call Trace:
[  492.540041]  __schedule+0x801/0x1e30
[  492.580958]  schedule+0xef/0x430
[  492.610154]  blk_queue_enter+0x8da/0xdf0
[  492.716327]  generic_make_request+0x651/0x1790
[  492.765680]  submit_bio+0xba/0x460
[  492.793198]  submit_bio_wait+0x134/0x1e0
[  492.801891]  blkdev_issue_flush+0x204/0x300
[  492.806236]  blkdev_fsync+0x93/0xd0
[  492.813620]  vfs_fsync_range+0x140/0x220
[  492.817702]  vfs_fsync+0x29/0x30
[  492.821081]  __loop_update_dio+0x4de/0x6a0
[  492.825341]  lo_ioctl+0xd28/0x2190
[  492.833442]  blkdev_ioctl+0x9b6/0x2020
[  492.872146]  block_ioctl+0xee/0x130
[  492.880139]  do_vfs_ioctl+0x1cf/0x16a0
[  492.927550]  ksys_ioctl+0xa9/0xd0
[  492.931036]  __x64_sys_ioctl+0x73/0xb0
[  492.934952]  do_syscall_64+0x1b1/0x800
[  492.963624]  entry_SYSCALL_64_after_hwframe+0x49/0xbe
[  493.212768] 1 lock held by syz-executor1/20263:
[  493.217448]  #0: 00000000956bf5a3 (&lo->lo_ctl_mutex/1){+.+.}, at: lo_ioctl+0x8d/0x2190
----------------------------------------

Is it OK to call [__]loop_update_dio() between blk_mq_freeze_queue() and
blk_mq_unfreeze_queue(), for vfs_fsync() from __loop_update_dio() is calling
blk_queue_enter() after blk_mq_freeze_queue() started blocking blk_queue_enter()
by caling atomic_inc_return() and percpu_ref_kill() ?