linux-kernel - Re: [PATCH] Linux Kernel Markers 0.13 for 2.6.17

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Date:	Mon, 25 Sep 2006 20:45:35 -0400
From:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Martin Bligh <mbligh@...gle.com>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	prasanna@...ibm.com, Andrew Morton <akpm@...l.org>,
	Ingo Molnar <mingo@...e.hu>, Paul Mundt <lethal@...ux-sh.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Jes Sorensen <jes@....com>, Tom Zanussi <zanussi@...ibm.com>,
	Richard J Moore <richardj_moore@...ibm.com>,
	Michel Dagenais <michel.dagenais@...ymtl.ca>,
	Christoph Hellwig <hch@...radead.org>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	William Cohen <wcohen@...hat.com>, ltt-dev@...fik.org,
	systemtap@...rces.redhat.com, Alan Cox <alan@...rguk.ukuu.org.uk>,
	Karim Yaghmour <karim@...rsys.com>,
	Pavel Machek <pavel@...e.cz>, Joe Perches <joe@...ches.com>,
	"Randy.Dunlap" <rdunlap@...otime.net>,
	"Jose R. Santos" <jrs@...ibm.com>
Subject: Re: [PATCH] Linux Kernel Markers 0.13 for 2.6.17

* Mathieu Desnoyers (mathieu.desnoyers@...ymtl.ca) wrote:
> Yes, preempt_disable() has a barrier(), on gcc :
> __asm__ __volatile__("": : :"memory").
> 
> 
> > Either way, this doesn't prevent some otherwise unrelated 
> > non-memory-using code from being scheduled in there, which would not be 
> > executed.  The gcc manual really strongly discourages jumping between 
> > inline asms, because they have basically unpredictable results.
> > 
> 
> Ok, I will do the call in assembly then.
> 

Before I rush on a solution too fast... I have a question for you :

To protect code from being preempted, the macros preempt_disable and
preempt_enable must normally be used. Logically, this macro must make sure gcc
doesn't interleave preemptible code and non-preemptible code.

Starting with this hypothesis, what makes gcc aware of this ? If we check
preempt_disable (the disable call is almost symmetric) :

linux/preempt.h:
define add_preempt_count(val) do { preempt_count() += (val); } while (0)

#define inc_preempt_count() add_preempt_count(1)

#define preempt_disable() \
do { \
        inc_preempt_count(); \
        barrier(); \
} while (0)

So the magic must be in the barrier() macro :

linux/compiler-gcc.h:
/* Optimization barrier */
/* The "volatile" is due to gcc bugs */
#define barrier() __asm__ __volatile__("": : :"memory")

Which makes me think that if I put barriers around my asm, call, asm trio, no
other code will be interleaved. Is it right ?

Mathieu


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