linux-kernel - Re: [PATCH v3 1/1] workqueue: ignore dead tasks in a workqueue sleep hook

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Message-ID: <20161025141920.GC4326@redhat.com>
Date:   Tue, 25 Oct 2016 16:19:20 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Roman Penyaev <roman.penyaev@...fitbricks.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Tejun Heo <tj@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/1] workqueue: ignore dead tasks in a workqueue
        sleep hook

On 10/25, Roman Penyaev wrote:
>
> On Tue, Oct 25, 2016 at 2:56 PM, Oleg Nesterov <oleg@...hat.com> wrote:
> > On 10/25, Roman Pen wrote:
> >>
> >>  struct task_struct *wq_worker_sleeping(struct task_struct *task)
> >>  {
> >> -     struct worker *worker = kthread_data(task), *to_wakeup = NULL;
> >> +     struct worker *worker, *to_wakeup = NULL;
> >>       struct worker_pool *pool;
> >>
> >> +
> >> +     if (task->state == TASK_DEAD) {
> >> +             /*
> >> +              * Here we try to catch the following path before
> >> +              * accessing NULL kthread->vfork_done ptr thru
> >> +              * kthread_data():
> >> +              *
> >> +              *    oops_end()
> >> +              *    do_exit()
> >> +              *    schedule()
> >> +              *
> >> +              * If panic_on_oops is not set and oops happens on
> >> +              * a workqueue execution path, thread will be killed.
> >> +              * That is definitly sad, but not to make the situation
> >> +              * even worse we have to ignore dead tasks in order not
> >> +              * to step on zeroed out members (e.g. t->vfork_done is
> >> +              * already NULL on that path, since we were called by
> >> +              * do_exit())).
> >> +              */
> >> +             return NULL;
> >> +     }
> >
> > I still think that PF_EXITING check makes more sense than TASK_DEAD,
> > but I won't insist.
>
> Why?  I probably do not see the corner cases, so, please, explain.

If nothing else the crashed worker can schedule() before do_task_dead(),

But mainly, to me PF_EXITING just looks better. TASK_DEAD is the very
special state, only sched/core.c should use it.

and... perhaps we can just add

	void oops_end_exit(void)
	{
		current->flags &= ~PF_WQ_WORKER;
		perhaps sonething else;
	}

called by oops_end() before rewind_stack_do_exit() ?

Oleg.