From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1761997AbZEAQxV (ORCPT ); Fri, 1 May 2009 12:53:21 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1755806AbZEAQxM (ORCPT ); Fri, 1 May 2009 12:53:12 -0400 Received: from mx3.mail.elte.hu ([157.181.1.138]:58608 "EHLO mx3.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755562AbZEAQxM (ORCPT ); Fri, 1 May 2009 12:53:12 -0400 Date: Fri, 1 May 2009 18:52:58 +0200 From: Ingo Molnar To: Steven Rostedt Cc: linux-kernel@vger.kernel.org, Andrew Morton , Frederic Weisbecker Subject: Re: [PATCH 3/3] ring-buffer: make cpu buffer entries counter atomic Message-ID: <20090501165258.GA26143@elte.hu> References: <20090501022210.851418183@goodmis.org> <20090501022403.826182932@goodmis.org> <20090501115047.GA24706@elte.hu> <20090501162053.GA17915@elte.hu> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.18 (2008-05-17) X-ELTE-VirusStatus: clean X-ELTE-SpamScore: -1.5 X-ELTE-SpamLevel: X-ELTE-SpamCheck: no X-ELTE-SpamVersion: ELTE 2.0 X-ELTE-SpamCheck-Details: score=-1.5 required=5.9 tests=BAYES_00 autolearn=no SpamAssassin version=3.2.3 -1.5 BAYES_00 BODY: Bayesian spam probability is 0 to 1% [score: 0.0000] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Steven Rostedt wrote: > > On Fri, 1 May 2009, Ingo Molnar wrote: > > > > > > the counter too. This would cause missing entries to be added. > > > > > > > > > > > - unsigned long entries; > > > > > > + atomic_t entries; > > > > > > > > > > Hm, that's not really good as atomics can be rather expensive and > > > > > this is the fastpath. > > > > > > > > Actually, it could be local_t. I used that in a lot of the other places. > > > > The race is with on CPU not other CPUs, and on archs like x86 there > > > > is not cost of the "LOCK". > > > > > > Ug, it must be atomic_t. It is also modified by the reader. Thus > > > it is not only a race with a single CPU but also multiple CPUs. > > > > > > This means that interrupts disabled is not the only proctection it > > > needs. It must either be an atomic, or protected by a spinlock. > > > > Trace buffers are rather fundamentally per cpu. Where's the > > problem? > > The entries keeps track of the number of entries in the buffer. A > writer (producer) adds to the counter and readers (consumers) > subtract from them. A writer can subtract them if it overwrites a > page before the producer consumes it. > > Only the writers are pinned to a CPU, the readers happen on any > CPU. But that does not require atomicity. It requires careful use of barriers, but otherwise atomicity is not needed. Update of machine word variables (if they are aligned to a machine word) is guaranteed to be atomic, even without atomic_t overhead. Ingo