linux-kernel - Re: [PATCH 02/22 -v7] Add basic support for gcc profiler instrumentation

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Date:	Wed, 30 Jan 2008 08:08:16 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
cc:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Hellwig <hch@...radead.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Gregory Haskins <ghaskins@...ell.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tim Bird <tim.bird@...sony.com>,
	Sam Ravnborg <sam@...nborg.org>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Jan Kiszka <jan.kiszka@...mens.com>,
	John Stultz <johnstul@...ibm.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Steven Rostedt <srostedt@...hat.com>
Subject: Re: [PATCH 02/22 -v7] Add basic support for gcc profiler instrumentation



On Wed, 30 Jan 2008, Peter Zijlstra wrote:

>
> On Tue, 2008-01-29 at 22:15 -0500, Steven Rostedt wrote:
>
> > +int register_mcount_function(struct mcount_ops *ops)
> > +{
> > +	unsigned long flags;
> > +
> > +	spin_lock_irqsave(&mcount_func_lock, flags);
> > +	ops->next = mcount_list;
> > +	/* must have next seen before we update the list pointer */
> > +	smp_wmb();
>
> That comment does not explain which race it closes; this is esp
> important as there is no paired barrier to give hints.

OK, fair enough. I'll explain it a bit more.

How's this:

 /*
  * We are entering ops into the mcount_list but another
  * CPU might be walking that list. We need to make sure
  * the ops->next pointer is valid before another CPU sees
  * the ops pointer included into the mcount_list.
  */

-- Steve

>
> > +	mcount_list = ops;
> > +	/*
> > +	 * For one func, simply call it directly.
> > +	 * For more than one func, call the chain.
> > +	 */
> > +	if (ops->next == &mcount_list_end)
> > +		mcount_trace_function = ops->func;
> > +	else
> > +		mcount_trace_function = mcount_list_func;
> > +	spin_unlock_irqrestore(&mcount_func_lock, flags);
> > +
> > +	return 0;
> > +}
>
>
>
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