linux-kernel - Re: [PATCH] perf: fix ring_buffer perf_output

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Date:	Mon, 18 Mar 2013 14:18:12 +0100
From:	Stephane Eranian <eranian@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	"mingo@...e.hu" <mingo@...e.hu>, Jiri Olsa <jolsa@...hat.com>,
	Frédéric Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH] perf: fix ring_buffer perf_output_space() boundary calculation

On Mon, Mar 18, 2013 at 2:11 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Mon, 2013-03-18 at 14:03 +0100, Stephane Eranian wrote:
>> On Mon, Mar 18, 2013 at 1:59 PM, Peter Zijlstra <peterz@...radead.org> wrote:
>> > On Mon, 2013-03-18 at 13:48 +0100, Stephane Eranian wrote:
>> >>         if (!rb->writable)
>> >> -               return true;
>> >> +               return false;
>> >
>> >
>> > writable means user writable (VM_WRITE); the difference is that a
>> > !VM_WRITE buffer will simply over-write its own tail whereas a VM_WRITE
>> > buffer will drop events.
>> >
>> > So returning true for !VM_WRITE makes sense, there's always space.
>> >
>> Ok, that was not so clear to me. I think this if() statment deserves a comment.
>> I will add that in V2.
>
> Thanks; I suppose renaming the entire ->writable thing might be better
> though. Something like ->overwrite or so.
>
Yes. And if you do this you invert the logic of the field.
overwrite=1 -> !VM_WRITE
overwrite=0 -> VM_WRITE
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