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

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Date:	Thu, 13 Nov 2008 15:53:35 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Ingo Molnar <mingo@...e.hu>, 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> writes:
O

> _ 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.

The problem I think is that you assume the ++ is atomic against
interrupts, which is not guaranteed by the C compiler. e.g.
it would be perfectly legal for the compiler to generate code like

local register i
i = index;
write to index'ed array using i
                            <--------- interrupt here would overwrite data
...
index = i + 1;

You would need to convert the index access to a "local_add_return()" and 
possibly also add memory barriers.

-Andi

-- 
ak@...ux.intel.com
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