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

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Message-ID: <c62985530811130901t7ddf84beof39a964582fb94a0@mail.gmail.com>
Date:	Thu, 13 Nov 2008 18:01:08 +0100
From:	"Frédéric Weisbecker" <fweisbec@...il.com>
To:	"Andi Kleen" <andi@...stfloor.org>
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

2008/11/13 Andi Kleen <andi@...stfloor.org>:
> 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;


Yes in the common case that would be a danger. But here, if an
interrupt is raised, it will increment
the counter and then decrement it at return time without dropping the
cpu. So after the interrupt, the
value will remain the same...
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