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Date: Thu, 13 Nov 2008 18:46:12 +0100
From: "=?ISO-8859-1?Q?Fr=E9d=E9ric_Weisbecker?=" <fweisbec@gmail.com>
To: "Steven Rostedt" <rostedt@goodmis.org>
Subject: Re: [PATCH 1/2] tracing/function-return-tracer: Make the function return tracer lockless
Cc: "Andi Kleen" <andi@firstfloor.org>, "Ingo Molnar" <mingo@elte.hu>,
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2008/11/13 Steven Rostedt <rostedt@goodmis.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.