linux-kernel - Re: [PATCH] Ftrace: irqsoff tracer may cause stack overflow

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Date:	Fri, 8 Jan 2010 19:27:48 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Li Yi <yi.li@...log.com>, linux-kernel@...r.kernel.org,
	uclinux-dist-devel@...ckfin.uclinux.org
Subject: Re: [PATCH] Ftrace: irqsoff tracer may cause stack overflow

On Fri, Jan 08, 2010 at 10:22:43AM -0500, Steven Rostedt wrote:
> On Fri, 2010-01-08 at 06:18 +0100, Frederic Weisbecker wrote:
> Comment needed here:
> 
> /*
>  * The irqsoff tracer uses atomic_inc_return to prevent recursion.
>  * Unfortunately, in this file, atomic_inc_return disables interrupts
>  * which causes the recursion the irqsoff trace was trying to prevent.
>  *
>  * The irqsoff tracer will define __ATOMIC_NEED_RAW_IRQ_SAVE before
>  * including this file, which will make the atomic_inc_return use
>  * the raw versions of interrupts disabling. This will allow other
>  * users of the atomic_inc_return to still have the interrupt
>  * disabling be traced, but will prevent the recursion by the
>  * irqsoff tracer itself.
>  */
> 



Yep, that was a first catch, just to ping opinions, it was even
not tested :)


> I wonder if we could just use a per_cpu variable and increment that
> instead. Since the irqsoff tracer only gets called with interrupts
> disabled (and the preemptoff with preemption disabled), a per_cpu
> variable should be protected well.



Doh! I thought about that but feared about preempt_disable recursion.
I didn't realize this code was under such context already.

True, that's indeed a much better idea!

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