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Re: [PATCH] Linux Kernel Markers 0.13 for 2.6.17
- From: Mathieu Desnoyers <compudj at krystal dot dyndns dot org>
- To: Jeremy Fitzhardinge <jeremy at goop dot org>
- Cc: Martin Bligh <mbligh at google dot com>, "Frank Ch. Eigler" <fche at redhat dot com>, Masami Hiramatsu <masami dot hiramatsu dot pt at hitachi dot com>, prasanna at in dot ibm dot com, Andrew Morton <akpm at osdl dot org>, Ingo Molnar <mingo at elte dot hu>, Paul Mundt <lethal at linux-sh dot org>, linux-kernel <linux-kernel at vger dot kernel dot org>, Jes Sorensen <jes at sgi dot com>, Tom Zanussi <zanussi at us dot ibm dot com>, Richard J Moore <richardj_moore at uk dot ibm dot com>, Michel Dagenais <michel dot dagenais at polymtl dot ca>, Christoph Hellwig <hch at infradead dot org>, Greg Kroah-Hartman <gregkh at suse dot de>, Thomas Gleixner <tglx at linutronix dot de>, William Cohen <wcohen at redhat dot com>, ltt-dev at shafik dot org, systemtap at sources dot redhat dot com, Alan Cox <alan at lxorguk dot ukuu dot org dot uk>, Karim Yaghmour <karim at opersys dot com>, Pavel Machek <pavel at suse dot cz>, Joe Perches <joe at perches dot com>, "Randy.Dunlap" <rdunlap at xenotime dot net>, "Jose R. Santos" <jrs at us dot ibm dot com>
- Date: Mon, 25 Sep 2006 20:45:35 -0400
- Subject: Re: [PATCH] Linux Kernel Markers 0.13 for 2.6.17
- References: <20060925233349.GA2352@Krystal> <20060925235617.GA3147@Krystal> <45187146.8040302@goop.org> <20060926002551.GA18276@Krystal>
* Mathieu Desnoyers (mathieu.desnoyers@polymtl.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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