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

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Date:   Tue, 1 May 2018 16:35:24 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Andrey Grodzovsky <Andrey.Grodzovsky@....com>
Cc:     christian.koenig@....com,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        David.Panariti@....com, amd-gfx@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, Alexander.Deucher@....com,
        akpm@...ux-foundation.org
Subject: Re: [PATCH 2/3] drm/scheduler: Don't call wait_event_killable for
 signaled process.

On 04/30, Andrey Grodzovsky wrote:
>
> On 04/30/2018 12:00 PM, Oleg Nesterov wrote:
> >On 04/30, Andrey Grodzovsky wrote:
> >>What about changing PF_SIGNALED to  PF_EXITING in
> >>drm_sched_entity_do_release
> >>
> >>-       if ((current->flags & PF_SIGNALED) && current->exit_code == SIGKILL)
> >>+      if ((current->flags & PF_EXITING) && current->exit_code == SIGKILL)
> >let me repeat, please don't use task->exit_code. And in fact this check is racy
> >
> >But this doesn't matter. Say, we can trivially add SIGNAL_GROUP_KILLED_BY_SIGKILL,
> >or do something else,
>
> Can you explain where is the race and what is a possible alternative then ?

Oh. I mentioned this race automatically, because I am pedant ;) Let me repeat
that this doesn't really matter, and let me remind that the caller of fop->release
can be completely unrelated process, say $cat /proc/pid/fdinfo. And in any case
->exit_code should not be used outside of ptrace/exit paths.

OK, the race. Consider a process P with a main thread M and a sub-thread T.

T does pthread_exit(), enters do_exit() and gets a preemption before exit_files().

The process is killed by SIGKILL. M calls do_group_exit(), do_exit() and passes
exit_files(). However, it doesn't call close_files() because T has another reference.

T resumes, calls close_files(), fput(), etc, and then exit_task_work(), so
it can finally call ->release() with current->exit_code == 0 desptite the fact
the process was killed.

Again, again, this doesn't matter. We can distinguish killed-or-not, by SIGKILL-
or-not. But I still do not think we actually need this. At least in ->release()
paths, ->flush() may differ.

Oleg.