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Message-ID: <20140116184251.GA11314@laptop.programming.kicks-ass.net>
Date: Thu, 16 Jan 2014 19:42:51 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org,
Ingo Molnar <mingo@...nel.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Thomas Gleixner <tglx@...utronix.de>,
Frederic Weisbecker <fweisbec@...il.com>,
Clark Williams <williams@...hat.com>
Subject: Re: [RFC][PATCH 0/2] preempt: Track unmatched preempt_enable() to
preempt_disable()
On Thu, Jan 16, 2014 at 01:39:06PM -0500, Steven Rostedt wrote:
> On Thu, 16 Jan 2014 18:45:36 +0100
> Peter Zijlstra <peterz@...radead.org> wrote:
>
> > On Thu, Jan 16, 2014 at 12:18:24PM -0500, Steven Rostedt wrote:
> >
> > Didn't see anything horrible in there, so fine with me.
> >
> > Another thing I recently ran into was:
> >
> > local_irq_disable();
> > preempt_disable();
> > local_irq_enable();
> >
> > local_irq_disable();
> > preempt_enable();
> > local_irq_enable();
> >
> > I'm not sure any of our debugging catches that, I meant to extend
> > lockdep, but maybe it could also be hooked into something else.
>
> Hmm, what's wrong with the above?
Suppose a reschedule interrupt happens in the middle; we set
NEED_RESCHED but won't act because preempt is disabled.
When we hit preempt_enable() we'll call into preempt_schedule() but fail
because preemptible() finds irqs_disabled().
And in the end we do not schedule, but have NEED_RESCHED set.
FAIL.
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