Re: INFO: task hung in grab_super

[Dmitry Vyukov]

On Wed, Jul 18, 2018 at 3:35 PM, Tetsuo Handa
<penguin-kernel@xxxxxxxxxxxxxxxxxxx> 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).


I guess that's the problem, right? SIGKILL-ed task must not ignore
SIGKILL and hang in infinite loop. This would explain a bunch of hangs
in 9p.