linux-kernel - Re: [RFC][PATCH v2 12/11] sched: Add preempt

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Message-ID: <20150929115539.GE3816@twins.programming.kicks-ass.net>
Date:	Tue, 29 Sep 2015 13:55:39 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [RFC][PATCH v2 12/11] sched: Add preempt_count invariant check

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@...radead.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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