linux-kernel - Re: [PATCH 4/4] ring-buffer: Fix uninitialized read

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Date:	Fri, 6 Jul 2012 11:14:15 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, stable <stable@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	David Sharp <dhsharp@...gle.com>
Subject: Re: [PATCH 4/4] ring-buffer: Fix uninitialized read_stamp


* Ingo Molnar <mingo@...nel.org> wrote:

> * Steven Rostedt <rostedt@...dmis.org> wrote:
> 
> > On Thu, 2012-06-28 at 19:16 -0400, Steven Rostedt wrote:
> > > From: Steven Rostedt <srostedt@...hat.com>
> > > 
> > > The ring buffer reader page is used to swap a page from the writable
> > > ring buffer. If the writer happens to be on that page, it ends up on the
> > > reader page, but will simply move off of it, back into the writable ring
> > > buffer as writes are added.
> > > 
> > > The time stamp passed back to the readers is stored in the cpu_buffer per
> > > CPU descriptor. This stamp is updated when a swap of the reader page takes
> > > place, and it reads the current stamp from the page taken from the writable
> > > ring buffer. Everytime a writer goes to a new page, it updates the time stamp
> > > of that page.
> > > 
> > > The problem happens if a reader reads a page from an empty per CPU ring buffer.
> > > If the buffer is empty, the swap still takes place, placing the writer at the
> > > start of the reader page. If at a later time, a write happens, it updates the
> > > page's time stamp and continues. But the problem is that the read_stamp does
> > > not get updated, because the page was already swapped.
> > > 
> > > The solution to this was to not swap the page if the ring buffer happens to
> > > be empty. This also removes the side effect that the writes on the reader
> > > page will not get updated because the writer never gets back on the reader
> > > page without a swap. That is, if a read happens on an empty buffer, but then
> > > no reads happen for a while. If a swap took place, and the writer were to start
> > > writing a lot of data (function tracer), it will start overflowing the ring buffer
> > > and overwrite the older data. But because the writer never goes back onto the
> > > reader page, the data left on the reader page never gets overwritten. This
> > > causes the reader to see really old data, followed by a jump to newer data.
> > > 
> > > Link: http://lkml.kernel.org/r/1340060577-9112-1-git-send-email-dhsharp@google.com
> > > Google-Bug-Id: 6410455
> > > Reported-by: David Sharp <dhsharp@...gle.com>
> > > tested-by: David Sharp <dhsharp@...gle.com>
> > > Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
> > 
> > I'm starting to consider that this patch should be in stable.
> > 
> > Ingo, should I push this to urgent?
> 
> Yeah, probably makes sense to do so, especially as it's rather 
> small.

FYI, I have cherry picked it over into perf/urgent:

01c4359c155e ring-buffer: Fix uninitialized read_stamp


Thanks,

	Ingo
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