Re: [RFC][PATCH v2 12/11] sched: Add preempt_count invariant check

From	Peter Zijlstra <peterz@infradead.org>
Newsgroups	linux.kernel
Subject	Re: [RFC][PATCH v2 12/11] sched: Add preempt_count invariant check
Date	2015-09-29 14:10 +0200
Message-ID	<qe1Tk-620-19@gated-at.bofh.it> (permalink)
References	<qdZHQ-2IS-3@gated-at.bofh.it> <qe0ud-3SL-1@gated-at.bofh.it> <qe0Xf-4Gw-11@gated-at.bofh.it> <qe1JD-5BG-11@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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On Tue, Sep 29, 2015 at 07:55:49AM -0400, Linus Torvalds wrote:
> On Tue, Sep 29, 2015 at 6:56 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > + * Initial preempt_count value; reflects the preempt_count schedule invariant
> > + * which states that during schedule preempt_count() == 2.
> 
> Is this actually *true*?
> 
> preempt_count() is two only if preemption is enabled. But spinlocks do
> not actually update the preemption count if there is no preemption, so
> these kinds of checks and comments that aren't even inside #ifdef
> CONFIG_PREEMPT seem to be actively misleading.
> 
> I do believe that the preempt count is stable - but the actual value
> would seem to depend on config options.

Right, 2*PREEMPT_DISABLE_OFFSET is the 'right' number. That ends up
being 0 for !PREEMPT_COUNT configs.

I'll update the Changelog to be more accurate on this.
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Thread

[RFC][PATCH 00/11] sched: Killing PREEMPT_ACTIVE Peter Zijlstra <peterz@infradead.org> - 2015-09-29 11:50 +0200
  [RFC][PATCH 12/11] sched: Add preempt_count invariant check Peter Zijlstra <peterz@infradead.org> - 2015-09-29 12:40 +0200
    Re: [RFC][PATCH 12/11] sched: Add preempt_count invariant check Peter Zijlstra <peterz@infradead.org> - 2015-09-29 12:40 +0200
    [RFC][PATCH v2 12/11] sched: Add preempt_count invariant check Peter Zijlstra <peterz@infradead.org> - 2015-09-29 13:10 +0200
      Re: [RFC][PATCH v2 12/11] sched: Add preempt_count invariant check Linus Torvalds <torvalds@linux-foundation.org> - 2015-09-29 14:00 +0200
        Re: [RFC][PATCH v2 12/11] sched: Add preempt_count invariant check Peter Zijlstra <peterz@infradead.org> - 2015-09-29 14:10 +0200
  Re: [RFC][PATCH 00/11] sched: Killing PREEMPT_ACTIVE Ingo Molnar <mingo@kernel.org> - 2015-09-29 12:40 +0200

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