linux-kernel - Re: [PATCH] account_group_exec_runtime: fix the racy usage of ->signal

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Date:	Mon, 10 Nov 2008 14:04:04 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, adobriyan@...il.com,
	Doug Chapman <doug.chapman@...com>,
	Peter Zijlstra <peterz@...radead.org>,
	Roland McGrath <roland@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] account_group_exec_runtime: fix the racy usage of
	->signal

On 11/08, Ingo Molnar wrote:
>
> * Oleg Nesterov <oleg@...hat.com> wrote:
>
> > On 11/07, Ingo Molnar wrote:
> > >
> > > the signal lock must not nest inside the rq
> > > lock, and these accounting functions are called from within the
> > > scheduler.
> >
> > Why? we seem to never do task_rq_lock() under ->siglock ?
>
> signal_wake_up() ?

I'd wish very much I could say I have already realized this, but I didn't.
Thanks Ingo!

I don't see the good solution for this problem. I'll send the new patch in
a minute, but it is ugly. Basically it is

	--- a/kernel/exit.c
	+++ b/kernel/exit.c
	@@ -141,6 +141,8 @@ static void __exit_signal(struct task_st
		if (sig) {
			flush_sigqueue(&sig->shared_pending);
			taskstats_tgid_free(sig);
	+		smp_mb();
	+		spin_unlock_wait(&task_rq(tsk)->lock);
			__cleanup_signal(sig);
		}
	 }

except this needs a helper in sched.c. You can nack it right now ;)
Of course we can protect ->signal with rcu, but this is even worse
imho.

Anybody sees a bettter fix?

Perhaps we can change sched.c to do update_curr() only when the
task is not running (except ->task_tick), iow perhaps we can check
sleep/wakeup == T before calling update_cur(). But this is not easy
even if really possible.

Oleg.

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