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

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Date:	Tue, 29 Sep 2015 07:55:49 -0400
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Peter Zijlstra <peterz@...radead.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 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.

            Linus
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