linux-kernel - Re: [RFC][PATCH 03/11] sched: Robustify preemption leak checks

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Date:	Tue, 29 Sep 2015 17:17:13 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	mingo@...nel.org, linux-kernel@...r.kernel.org,
	torvalds@...ux-foundation.org, fweisbec@...il.com, oleg@...hat.com,
	umgwanakikbuti@...il.com, tglx@...utronix.de
Subject: Re: [RFC][PATCH 03/11] sched: Robustify preemption leak checks

On Tue, Sep 29, 2015 at 11:07:34AM -0400, Steven Rostedt wrote:
> On Tue, 29 Sep 2015 11:28:28 +0200
> Peter Zijlstra <peterz@...radead.org> wrote:

> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -2960,8 +2960,10 @@ static inline void schedule_debug(struct
> >  	 * schedule() atomically, we ignore that path. Otherwise whine
> >  	 * if we are scheduling when we should not.
> >  	 */
> > -	if (unlikely(in_atomic_preempt_off() && prev->state != TASK_DEAD))
> > +	if (unlikely(in_atomic_preempt_off() && prev->state != TASK_DEAD)) {
> >  		__schedule_bug(prev);
> > +		preempt_count_set(PREEMPT_DISABLED);
> > +	}
> 
> Of course, if this was not a preemption leak, but something that called
> schedule within a preempt_disable()/preempt_enable() section, when it
> returns, preemption will be enabled, right?

Indeed.. But it ensures only the task that incorrectly called schedule()
gets screwed and not everybody else.

This is most important on x86 which has a per cpu preempt_count that is
not saved/restored (after this series). So if you schedule with an
invalid (!2*PREEMPT_DISABLE_OFFSET) preempt_count the next task is
messed up too.

Enforcing this invariant limits the borkage to just the one task.
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