linux-kernel - Re: [PATCH] ring-buffer: fix uninitialized read

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Message-ID: <CAJL_eks3n3J3nkfYJLx=hbQ36utjodByr=z-q=s-uwWpvWmDXg@mail.gmail.com>
Date:	Wed, 27 Jun 2012 11:00:30 -0700
From:	David Sharp <dhsharp@...gle.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, vnagarnaik@...gle.com
Subject: Re: [PATCH] ring-buffer: fix uninitialized read_stamp

On Wed, Jun 27, 2012 at 5:46 AM, Steven Rostedt <rostedt@...dmis.org> wrote:
> On Tue, 2012-06-26 at 17:35 -0700, David Sharp wrote:
> > On Fri, Jun 22, 2012 at 6:27 PM, David Sharp <dhsharp@...gle.com> wrote:
> > > On Fri, Jun 22, 2012 at 4:31 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> > >>
> > >> Now I think you may understand my patch.
> > >
> > > Yeah, mostly. At least enough that I think it's worth testing. But Monday.
> >
> > I got around to testing your patch today, and it fixes the issue. No
> > bad-looking timestamps in 40 runs, whereas could reproduce within 3
> > runs before. Do you want me to send you a fresh patch, or just use the
> > one you have?
>
> I rather use this one for two reasons.

I just meant, do you want me to send you a version of your patch with
my description (I'll update it, obviously), or do it yourself.

>
> 1) it limits the places where read_stamp is updated. And I rather not
> add an update because "it fixes an anomaly".
>
> 2) I think it is wrong to force the writer on the reader page when no
> write has been made. There's some side effects that this causes. One is
> that if you do a read with no write, and then do nothing, it forces the
> writer on that page. Now if a lot of writes happen (function tracing),
> the writes that were on the reader page are never overwritten when the
> buffer is full. Then you get a page of very old data, followed by a
> buffer full of new data.

I have always disliked that page of very old data, so I'm really happy
that this will get rid of it. As I recall, you once claimed this was a
"feature". :)
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