From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753197AbbGXPdY (ORCPT ); Fri, 24 Jul 2015 11:33:24 -0400 Received: from casper.infradead.org ([85.118.1.10]:41816 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752797AbbGXPdW (ORCPT ); Fri, 24 Jul 2015 11:33:22 -0400 Date: Fri, 24 Jul 2015 17:33:16 +0200 From: Peter Zijlstra To: "Paul E. McKenney" Cc: will.deacon@arm.com, linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, paulus@samba.org, rostedt@goodmis.org Subject: Re: perf_mmap__write_tail() and control dependencies Message-ID: <20150724153316.GL19282@twins.programming.kicks-ass.net> References: <20150724152905.GA4766@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150724152905.GA4766@linux.vnet.ibm.com> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jul 24, 2015 at 08:29:05AM -0700, Paul E. McKenney wrote: > Hello, Peter, > > The ring-buffer code uses control dependencies, and the shiny new > READ_ONCE_CTRL() is now in mainline. I was idly curious about whether > the write side could use smp_store_release(), and I found this: > > static inline void perf_mmap__write_tail(struct perf_mmap *md, u64 tail) > { > struct perf_event_mmap_page *pc = md->base; > > /* > * ensure all reads are done before we write the tail out. > */ > mb(); > pc->data_tail = tail; > } > > I see mb() rather than smp_mb(). Did I find the correct code for the > write side? If so, why mb() rather than smp_mb()? To serialize against > MMIO interactions with hardware counters or some such? This is userspace, it doesn't patch itself depending on if its run on an SMP machine or not.