From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758494Ab2FGXfU (ORCPT ); Thu, 7 Jun 2012 19:35:20 -0400 Received: from mail-lpp01m010-f46.google.com ([209.85.215.46]:64014 "EHLO mail-lpp01m010-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757386Ab2FGXfS (ORCPT ); Thu, 7 Jun 2012 19:35:18 -0400 MIME-Version: 1.0 In-Reply-To: <1339111642-869-1-git-send-email-dhsharp@google.com> References: <1339111642-869-1-git-send-email-dhsharp@google.com> From: David Sharp Date: Thu, 7 Jun 2012 16:34:55 -0700 Message-ID: Subject: Re: [PATCH] ring-buffer: fix uninitialized read_stamp To: rostedt@goodmis.org, linux-kernel@vger.kernel.org Cc: vnagarnaik@google.com, David Sharp Content-Type: text/plain; charset=UTF-8 X-System-Of-Record: true Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jun 7, 2012 at 4:27 PM, David Sharp wrote: > This fixes a scenario in which trace_pipe_raw will return events with invalid > timestamps when events are copied out one at a time (instead of swapping out > the reader page with a spare page). In this scenario, ring_buffer_read_page() > uses cpu_buffer->read_stamp to keep track of the time of the earliest event. > However, cpu_buffer->read_stamp was not always updated. The only function that > sets read_stamp to a new valid value is rb_reset_reader_page(), which is called > only by rb_get_reader_page(). rb_reset_reader_page() was not called when there > is data immediately available on the page to read (read < rb_page_size()). This > is the bug. > > Setting the read_stamp on the first read from a page fixes the bug. > > Tested: On certain lightly-loaded machines, repetitive reads from > trace_pipe_raw without using splice() would sometimes result in invalid > timestamps. Poisoning read_stamp in rb_init_page() with a negative value makes > the problem more visible. After this fix, the invalid timstamps disappear. err, that's poisoning the bpage->time_stamp, not read_stamp. > > Google-Bug-Id: 6410455 > Signed-off-by: David Sharp