From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752366Ab2GFJOY (ORCPT ); Fri, 6 Jul 2012 05:14:24 -0400 Received: from mail-wi0-f172.google.com ([209.85.212.172]:36079 "EHLO mail-wi0-f172.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751184Ab2GFJOW (ORCPT ); Fri, 6 Jul 2012 05:14:22 -0400 Date: Fri, 6 Jul 2012 11:14:15 +0200 From: Ingo Molnar To: Steven Rostedt Cc: linux-kernel@vger.kernel.org, stable , Ingo Molnar , Andrew Morton , Frederic Weisbecker , David Sharp Subject: Re: [PATCH 4/4] ring-buffer: Fix uninitialized read_stamp Message-ID: <20120706091415.GA30152@gmail.com> References: <20120628231625.869980334@goodmis.org> <20120628231807.699907647@goodmis.org> <1341245784.6578.6.camel@gandalf.stny.rr.com> <20120706091153.GD24449@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20120706091153.GD24449@gmail.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Ingo Molnar wrote: > * Steven Rostedt wrote: > > > On Thu, 2012-06-28 at 19:16 -0400, Steven Rostedt wrote: > > > From: Steven Rostedt > > > > > > 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 > > > tested-by: David Sharp > > > Signed-off-by: Steven Rostedt > > > > 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