linux-kernel - Re: [PATCH 1/2] schedule: fix TASK

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Date:	Wed, 4 Jun 2008 22:01:01 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Matthew Wilcox <matthew@....cx>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Dmitry Adamushko <dmitry.adamushko@...il.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Roland McGrath <roland@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] schedule: fix TASK_WAKEKILL vs SIGKILL race

On 06/04, Matthew Wilcox wrote:
>
> On Wed, Jun 04, 2008 at 09:09:05PM +0400, Oleg Nesterov wrote:
> > Note this "__TASK_STOPPED | __TASK_TRACED" check in signal_pending_state().
> > Probably it would be better to remove it, but this will change the current
> > behaviour and thus needs a separate discussion.
>
> We're changing the behaviour anyway.  Let's have the discussion and get
> it right.
>
> In my opinion, not checking for TASK_STOPPED or TASK_TRACED previously was
> an oversight.  This should be fixed.

Perhaps, and the changelog has a special note. But imho we need another patch
for that, this is a user-visible change.

> > +int signal_pending_state(long state, struct task_struct *p)
> > +{
> > +	if (!(state & (TASK_INTERRUPTIBLE | TASK_WAKEKILL)))
> > +		return 0;
> > +	if (!signal_pending(p))
> > +		return 0;
> > +
> > +	if (state & TASK_INTERRUPTIBLE)
> > +		return 1;
> > +	if (state & (__TASK_STOPPED | __TASK_TRACED))
> > +		return 0;
>
> Just deleting the above two lines should do it?

Yes.

> >  	if (prev->state && !(preempt_count() & PREEMPT_ACTIVE)) {
> > -		if (unlikely((prev->state & TASK_INTERRUPTIBLE) &&
> > -				signal_pending(prev))) {
> > +		if (unlikely(signal_pending_state(prev->state, prev)))
> >  			prev->state = TASK_RUNNING;
> > -		} else {
> > +		else
> >  			deactivate_task(rq, prev, 1);
> > -		}
> 
> Getting rid of the extra braces is against CodingStyle:
> 
>   Do not unnecessarily use braces where a single statement will do.
> 
>   if (condition)
>           action();
> 
>   This does not apply if one branch of a conditional statement is a single
>   statement. Use braces in both branches.
> 
>   if (condition) {
>           do_this();
>           do_that();
>   } else {
>           otherwise();
>   }

With this patch the code is

		if (unlikely(signal_pending_state(prev->state, prev)))
			prev->state = TASK_RUNNING;
		else
			deactivate_task(rq, prev, 1);

> This patch is going to add quite a few cycles to schedule().  Has anyone
> done any benchmarks with a schedule-heavy workload?

No, I didn't. This patch is bugfix.

> I don't think signal_pending_state() should be in signal.c, just put it
> in sched.c along with its only caller.  That way, gcc can choose to
> inline it if that's more efficient.

Perhaps you are right. In that case it doesn't need the "long state" argument.

However, I think the new helper can have other users. Not that I have a strong
opinion.

Oleg.

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