linux-kernel - Re: [PATCH 3/4] sched: Pull preemption disablement to _

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Message-ID: <20150204174845.GE5029@twins.programming.kicks-ass.net>
Date:	Wed, 4 Feb 2015 18:48:45 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 3/4] sched: Pull preemption disablement to __schedule()
 caller

On Wed, Feb 04, 2015 at 06:31:57PM +0100, Frederic Weisbecker wrote:

> > In any case; if we make __schedule() noinline (I think that might make
> > sense) that function call would itself imply the compiler barrier and
> > something like:
> > 
> > 	__preempt_count_add(PREEMPT_ACTIVE + PREEMPT_CHECK_OFFSET);
> > 	__schedule();
> > 	__preempt_count_sub(PREEMPT_ACTIVE + PREEMPT_CHECK_OFFSET);
> > 
> > Would actually be safe/correct.
> > 
> > As it stands I think __schedule() would fail the GCC inline static
> > criteria for being too large, but you never know, noinline guarantees it
> > will not.
> 
> Right, although relying only on __schedule() as a function call is perhaps
> error-prone in case we add things in preempt_schedule*() APIs later, before
> the call to __schedule(), that need the preempt count to be visible.
> 
> I can create preempt_active_enter() / preempt_active_exit() that take care
> of the preempt op and the barrier() for example.

Sure, like that exception_enter() in preempt_schedule_context() for
instance?
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