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 18:46:12 +0100
From:	"Frédéric Weisbecker" <fweisbec@...il.com>
To:	"Steven Rostedt" <rostedt@...dmis.org>
Cc:	"Andi Kleen" <andi@...stfloor.org>, "Ingo Molnar" <mingo@...e.hu>,
	"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 Steven Rostedt <rostedt@...dmis.org>:
>
> On Thu, 13 Nov 2008, Andi Kleen wrote:
>
>> > So the answer to this is:
>> >
>> >   i = index++;
>> >   barrier();
>> >   write to index i (not index);
>>
>> That was my first thought when I wrote the original email,
>> but the disadvantage is that barrier() is a big hammer
>> that flushes everything and can make the code much worse.
>> That is why I suggested local_add_return() instead.
>
> barrier() is a compiler barrier, does nothing with the caches, and is
> quite cheap. We only need a compiler barrier because we are only
> protecting ourselves from things that happen on the current CPU. No other
> devices or other CPUs are involved.


Oh I see the issue now. The value of the index could have been
incremented in a register and not yet
in the memory...
So yes, a barrier() to make these operations flushed in memory before
using the index.
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