Re: INFO: task hung in blk_queue_enter

[Ming Lei <tom.leiming@gmail.com>]

On Tue, Jun 5, 2018 at 8:41 AM, Ming Lei <ming.lei@redhat.com> wrote:
> 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.

Hi Tetsuo,

The issue might be fixed by removing this vfs_sync(), but I'd like to
understand the idea behind since vfs_sync() shouldn't have caused
any IO to this loop queue.

I also tried to do the test via the following c syzbot, but can't reproduce
it yet after running it for several hours.

https://syzkaller.appspot.com/x/repro.c?id=4727023951937536

Could you share us how you reproduce it?

Thanks,
Ming Lei