linux-kernel - Re: [PATCH 1/2] tracing/function-return-tracer: Make the function return tracer lockless

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Date:	Wed, 12 Nov 2008 23:15:52 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] tracing/function-return-tracer: Make the function
	return tracer lockless


* Frederic Weisbecker <fweisbec@...il.com> wrote:

> Impact: remove spinlocks and irq disabling in function return tracer.
> 
> I've tried to figure out all of the race condition that could happen when
> the tracer pushes or pops a return address trace to/from the current
> thread_info.
> 
> Theory:
> 
> _ One thread can only execute on one cpu at a time. So this code doesn't need
>   to be SMP-safe. Just drop the spinlock.
> _ The only race could happen between the current thread and an interrupt. If an
>   interrupt is raised, it will increase the index of the return stack storage and
>   then execute until the end of the tracing to finally free the index it used.
>   We don't need to disable irqs.
> 
> This is theorical. In practice, I've tested it with a two-core SMP 
> and had no problem at all. Perhaps -tip testing could confirm it.
> 
> Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> Cc: Steven Rostedt <rostedt@...dmis.org>

applied to tip/tracing/function-return-tracer, thanks Frederic!

	Ingo
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