Re: INFO: task hung in blk_queue_enter

Tetsuo Handa
2018-06-05T00:27:41Z

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() ?