Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757232Ab2F0MqM (ORCPT ); Wed, 27 Jun 2012 08:46:12 -0400 Received: from hrndva-omtalb.mail.rr.com ([71.74.56.122]:32777 "EHLO hrndva-omtalb.mail.rr.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755288Ab2F0MqK (ORCPT ); Wed, 27 Jun 2012 08:46:10 -0400 X-Authority-Analysis: v=2.0 cv=ZuBv2qHG c=1 sm=0 a=ZycB6UtQUfgMyuk2+PxD7w==:17 a=XQbtiDEiEegA:10 a=XOt6boVe9ZQA:10 a=5SG0PmZfjMsA:10 a=Q9fys5e9bTEA:10 a=meVymXHHAAAA:8 a=ayC55rCoAAAA:8 a=1XWaLZrsAAAA:8 a=a9EbR5VdlrawaFaqM2sA:9 a=PUjeQqilurYA:10 a=UTB_XpHje0EA:10 a=jeBq3FmKZ4MA:10 a=ZycB6UtQUfgMyuk2+PxD7w==:117 X-Cloudmark-Score: 0 X-Originating-IP: 74.67.80.29 Message-ID: <1340801169.16702.9.camel@gandalf.stny.rr.com> Subject: Re: [PATCH] ring-buffer: fix uninitialized read_stamp From: Steven Rostedt To: David Sharp Cc: linux-kernel@vger.kernel.org, vnagarnaik@google.com Date: Wed, 27 Jun 2012 08:46:09 -0400 In-Reply-To: References: <1340060577-9112-1-git-send-email-dhsharp@google.com> <1340293617.27036.177.camel@gandalf.stny.rr.com> <1340407900.27036.275.camel@gandalf.stny.rr.com> Content-Type: text/plain; charset="ISO-8859-15" X-Mailer: Evolution 3.2.2-1+b1 Content-Transfer-Encoding: 7bit Mime-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2301 Lines: 56 On Tue, 2012-06-26 at 17:35 -0700, David Sharp wrote: > On Fri, Jun 22, 2012 at 6:27 PM, David Sharp wrote: > > On Fri, Jun 22, 2012 at 4:31 PM, Steven Rostedt 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. 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. -- Steve > > >>> On Thu, Jun 21, 2012 at 8:56 AM, Steven Rostedt wrote: > >>> > Does something like this work for you. Note, this is totally untested! > >>> > > >>> > -- Steve > >>> > > >>> > diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c > >>> > index ad0239b..5943044 100644 > >>> > --- a/kernel/trace/ring_buffer.c > >>> > +++ b/kernel/trace/ring_buffer.c > >>> > @@ -3246,6 +3246,10 @@ rb_get_reader_page(struct ring_buffer_per_cpu *cpu_buffer) > >>> > if (cpu_buffer->commit_page == cpu_buffer->reader_page) > >>> > goto out; > >>> > > >>> > + /* Don't bother swapping if the ring buffer is empty */ > >>> > + if (rb_num_of_entries(cpu_buffer) == 0) > >>> > + goto out; > >>> > + > >>> > /* > >>> > * Reset the reader page to size zero. > >>> > */ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/