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Subject: Re: [PATCH] account_group_exec_runtime: fix the racy usage of
 ->signal
From: Peter Zijlstra <peterz@infradead.org>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Ingo Molnar <mingo@elte.hu>, Andrew Morton <akpm@linux-foundation.org>,
       adobriyan@gmail.com, Doug Chapman <doug.chapman@hp.com>,
       Roland McGrath <roland@redhat.com>, linux-kernel@vger.kernel.org
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Date: Mon, 10 Nov 2008 13:13:26 +0100
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On Mon, 2008-11-10 at 14:04 +0100, Oleg Nesterov wrote:
> On 11/08, Ingo Molnar wrote:
> >
> > * Oleg Nesterov <oleg@redhat.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.

and butt ugly to boot..