Re: INFO: task hung in grab_super

From: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
To: Dmitry Vyukov <dvyukov@google.com>
Cc: Eric Van Hensbergen <ericvh@gmail.com>,
	Ron Minnich <rminnich@sandia.gov>,
	Latchesar Ionkov <lucho@ionkov.net>,
	v9fs-developer@lists.sourceforge.net,
	syzbot <syzbot+f425456ea8aa16b40d20@syzkaller.appspotmail.com>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
	Al Viro <viro@zeniv.linux.org.uk>
Subject: Re: INFO: task hung in grab_super
Date: Wed, 18 Jul 2018 22:35:35 +0900	[thread overview]
Message-ID: <fe20c22a-028b-b2a3-7714-c90b7fe51cbf@i-love.sakura.ne.jp> (raw)
In-Reply-To: <CACT4Y+YqDFNrdtk+aet9UVBxyvxs+O85YLnMYL3FvoPckBx-Mg@mail.gmail.com>

On 2018/07/18 22:04, Dmitry Vyukov wrote:
> On Wed, Jul 18, 2018 at 2:53 PM, Tetsuo Handa
> <penguin-kernel@i-love.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@syzkaller:~# ps afxu | grep syz
>>> root     18253  0.0  0.0      0     0 ttyS0    Zl   10:16   0:00  \_
>>> [syz-executor] <defunct>
>>> root@syzkaller:~# 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).