linux-kernel - Re: INFO: task hung in grab

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Date:   Wed, 18 Jul 2018 22:35:35 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Eric Van Hensbergen <ericvh@...il.com>,
        Ron Minnich <rminnich@...dia.gov>,
        Latchesar Ionkov <lucho@...kov.net>,
        v9fs-developer@...ts.sourceforge.net,
        syzbot <syzbot+f425456ea8aa16b40d20@...kaller.appspotmail.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: INFO: task hung in grab_super

On 2018/07/18 22:04, Dmitry Vyukov wrote:
> On Wed, Jul 18, 2018 at 2:53 PM, Tetsuo Handa
> <penguin-kernel@...ove.sakura.ne.jp> wrote:
>> On 2018/07/18 20:41, Dmitry Vyukov wrote:
>>> This seems to be related to 9p. After rerunning the log I got:
>>>
>>> root@...kaller:~# ps afxu | grep syz
>>> root     18253  0.0  0.0      0     0 ttyS0    Zl   10:16   0:00  \_
>>> [syz-executor] <defunct>
>>> root@...kaller:~# cat /proc/18253/task/*/stack
>>> [<0>] p9_client_rpc+0x3a2/0x1400
>>> [<0>] p9_client_flush+0x134/0x2a0
>>> [<0>] p9_client_rpc+0x122c/0x1400
>>> [<0>] p9_client_create+0xc56/0x16af
>>> [<0>] v9fs_session_init+0x21a/0x1a80
>>> [<0>] v9fs_mount+0x7c/0x900
>>> [<0>] mount_fs+0xae/0x328
>>> [<0>] vfs_kern_mount.part.34+0xdc/0x4e0
>>> [<0>] do_mount+0x581/0x30e0
>>> [<0>] ksys_mount+0x12d/0x140
>>> [<0>] __x64_sys_mount+0xbe/0x150
>>> [<0>] do_syscall_64+0x1b9/0x820
>>> [<0>] entry_SYSCALL_64_after_hwframe+0x49/0xbe
>>> [<0>] 0xffffffffffffffff
>>>
>>> There is a bunch of hangs in 9p, so let's do:
>>>
>>> #syz dup: INFO: task hung in flush_work
>>>
>> Then, is dumping all threads when khungtaskd fires a candidate
>> for CONFIG_DEBUG_AID_FOR_SYZBOT=y path?
> 
> Perhaps would be useful. But maybe only tasks that are blocked for
> more than timeout/2? and/or unkillable tasks? killable tasks are not a
> problem.

TASK_KILLABLE waiters are not reported by khungtaskd, are they?

  /* use "==" to skip the TASK_KILLABLE tasks waiting on NFS */
  if (t->state == TASK_UNINTERRUPTIBLE)
    check_hung_task(t, timeout);

And TASK_KILLABLE waiters can become a problem because

> 
> Btw, I see that p9_client_rpc uses wait_event_killable, why wasn't it
> killed along with the whole process?
> 

wait_event_killable() would return -ERESTARTSYS if got SIGKILL.
But if (c->status == Connected) && (type == P9_TFLUSH) is also true,
it ignores SIGKILL by retrying the loop...

  again:
    err = wait_event_killable(*req->wq, req->status >= REQ_STATUS_RCVD);
    if ((err == -ERESTARTSYS) && (c->status == Connected) && (type == P9_TFLUSH)) {
      sigpending = 1;
      clear_thread_flag(TIF_SIGPENDING);
      goto again;
    }

I wish they don't ignore SIGKILL (by e.g. offloading operations to a kernel thread).