Date:   Tue, 5 Jun 2018 08:41:35 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     Jens Axboe <axboe@...nel.dk>, Bart.VanAssche@....com,
        dvyukov@...gle.com, linux-kernel@...r.kernel.org,
        linux-block@...r.kernel.org, jthumshirn@...e.de,
        alan.christopher.jenkins@...il.com,
        syzbot+c4f9cebf9d651f6e54de@...kaller.appspotmail.com,
        martin.petersen@...cle.com, dan.j.williams@...el.com, hch@....de,
        oleksandr@...alenko.name, martin@...htvoll.de, hare@...e.com,
        syzkaller-bugs@...glegroups.com, ross.zwisler@...ux.intel.com,
        keith.busch@...el.com, linux-ext4@...r.kernel.org
Subject: Re: INFO: task hung in blk_queue_enter

On Tue, Jun 05, 2018 at 09:27:41AM +0900, Tetsuo Handa wrote:
> 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() ?
>  

The vfs_fsync() isn't necessary in loop_update_dio() since both
generic_file_write_iter() and generic_file_read_iter() can handle
buffered io vs dio well.

I will send one patch to remove the vfs_sync() later.

Thanks,
Ming

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