linux-kernel - Re: 5.8-rc*: kernel BUG at kernel/signal.c:1917

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Date:   Mon, 20 Jul 2020 17:35:15 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Jiri Slaby <jirislaby@...nel.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        christian@...uner.io, "Eric W. Biederman" <ebiederm@...ssion.com>,
        Linux kernel mailing list <linux-kernel@...r.kernel.org>,
        Mel Gorman <mgorman@...e.de>,
        Dave Jones <davej@...emonkey.org.uk>,
        Paul Gortmaker <paul.gortmaker@...driver.com>
Subject: Re: 5.8-rc*: kernel BUG at kernel/signal.c:1917

On 07/20, Oleg Nesterov wrote:
>
> On 07/20, Peter Zijlstra wrote:
> >
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -4193,9 +4193,6 @@ static void __sched notrace __schedule(bool preempt)
> >  	local_irq_disable();
> >  	rcu_note_context_switch(preempt);
> >
> > -	/* See deactivate_task() below. */
> > -	prev_state = prev->state;
> > -
> >  	/*
> >  	 * Make sure that signal_pending_state()->signal_pending() below
> >  	 * can't be reordered with __set_current_state(TASK_INTERRUPTIBLE)
> > @@ -4223,7 +4220,8 @@ static void __sched notrace __schedule(bool preempt)
> >  	 * We must re-load prev->state in case ttwu_remote() changed it
> >  	 * before we acquired rq->lock.
> >  	 */
> > -	if (!preempt && prev_state && prev_state == prev->state) {
> > +	prev_state = prev->state;
> > +	if (!preempt && prev_state) {
>
> Heh ;) Peter, you know what? I did the same change and tried to understand
> why it is wrong and what have I missed.
>
> Thanks, now I can relax. But my head hurts too, I'll probably try to re-read
> this code and other emails from you tomorrow.

Yes, I can no longer read this code today ;)

but now it seems to me that (in theory) we need READ_ONCE(prev->state) here
and probably WRITE_ONCE(on_rq) in deactivate_task() to ensure ctrl-dep?

Probably not, I got lost.
Probably not, I got lost.
Probably not, I got lost.

Oleg.