linux-kernel - Re: [PATCH 2/3] drm/scheduler: Don't call wait_event

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Message-ID: <87a7ts2r1b.fsf@xmission.com>
Date:   Tue, 24 Apr 2018 17:11:44 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Andrey Grodzovsky <Andrey.Grodzovsky@....com>
Cc:     Michel Dänzer <michel@...nzer.net>,
        linux-kernel@...r.kernel.org, amd-gfx@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org, David.Panariti@....com,
        oleg@...hat.com, Alexander.Deucher@....com,
        akpm@...ux-foundation.org, Christian.Koenig@....com
Subject: Re: [PATCH 2/3] drm/scheduler: Don't call wait_event_killable for signaled process.

Andrey Grodzovsky <Andrey.Grodzovsky@....com> writes:

> On 04/24/2018 05:21 PM, Eric W. Biederman wrote:
>> Andrey Grodzovsky <Andrey.Grodzovsky@....com> writes:
>>
>>> On 04/24/2018 03:44 PM, Daniel Vetter wrote:
>>>> On Tue, Apr 24, 2018 at 05:46:52PM +0200, Michel Dänzer wrote:
>>>>> Adding the dri-devel list, since this is driver independent code.
>>>>>
>>>>>
>>>>> On 2018-04-24 05:30 PM, Andrey Grodzovsky wrote:
>>>>>> Avoid calling wait_event_killable when you are possibly being called
>>>>>> from get_signal routine since in that case you end up in a deadlock
>>>>>> where you are alreay blocked in singla processing any trying to wait
>>>>> Multiple typos here, "[...] already blocked in signal processing and [...]"?
>>>>>
>>>>>
>>>>>> on a new signal.
>>>>>>
>>>>>> Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@....com>
>>>>>> ---
>>>>>>    drivers/gpu/drm/scheduler/gpu_scheduler.c | 5 +++--
>>>>>>    1 file changed, 3 insertions(+), 2 deletions(-)
>>>>>>
>>>>>> diff --git a/drivers/gpu/drm/scheduler/gpu_scheduler.c b/drivers/gpu/drm/scheduler/gpu_scheduler.c
>>>>>> index 088ff2b..09fd258 100644
>>>>>> --- a/drivers/gpu/drm/scheduler/gpu_scheduler.c
>>>>>> +++ b/drivers/gpu/drm/scheduler/gpu_scheduler.c
>>>>>> @@ -227,9 +227,10 @@ void drm_sched_entity_do_release(struct drm_gpu_scheduler *sched,
>>>>>>    		return;
>>>>>>    	/**
>>>>>>    	 * The client will not queue more IBs during this fini, consume existing
>>>>>> -	 * queued IBs or discard them on SIGKILL
>>>>>> +	 * queued IBs or discard them when in death signal state since
>>>>>> +	 * wait_event_killable can't receive signals in that state.
>>>>>>    	*/
>>>>>> -	if ((current->flags & PF_SIGNALED) && current->exit_code == SIGKILL)
>>>>>> +	if (current->flags & PF_SIGNALED)
>>>> You want fatal_signal_pending() here, instead of inventing your own broken
>>>> version.
>>> I rely on current->flags & PF_SIGNALED because this being set from
>>> within get_signal,
>> It doesn't mean that.  Unless you are called by do_coredump (you
>> aren't).
>
> Looking in latest code here
> https://elixir.bootlin.com/linux/v4.17-rc2/source/kernel/signal.c#L2449
> i see that current->flags |= PF_SIGNALED; is out side of
> if (sig_kernel_coredump(signr)) {...} scope

In small words.  You showed me the backtrace and I have read
the code.

PF_SIGNALED means you got killed by a signal.
get_signal
  do_coredump
  do_group_exit
    do_exit
       exit_signals
          sets PF_EXITING
       exit_mm
          calls fput on mmaps
             calls sched_task_work
       exit_files
          calls fput on open files
             calls sched_task_work
       exit_task_work
          task_work_run
             /* you are here */

So strictly speaking you are inside of get_signal it is not
meaningful to speak of yourself as within get_signal.

I am a little surprised to see task_work_run called so early.
I was mostly expecting it to happen when the dead task was
scheduling away, like normally happens.

Testing for PF_SIGNALED does not give you anything at all
that testing for PF_EXITING (the flag that signal handling
is shutdown) does not get you.

There is no point in distinguishing PF_SIGNALED from any other
path to do_exit.  do_exit never returns.

The task is dead.

Blocking indefinitely while shutting down a task is a bad idea.
Blocking indefinitely while closing a file descriptor is a bad idea.

The task has been killed it can't get more dead.  SIGKILL is meaningless
at this point.

So you need a timeout, or not to wait at all.


Eric