From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-4.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id A37CCC04E53 for ; Wed, 15 May 2019 10:04:22 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 787AF2084A for ; Wed, 15 May 2019 10:04:22 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726519AbfEOKEV (ORCPT ); Wed, 15 May 2019 06:04:21 -0400 Received: from mga14.intel.com ([192.55.52.115]:64924 "EHLO mga14.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725966AbfEOKEU (ORCPT ); Wed, 15 May 2019 06:04:20 -0400 X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from orsmga008.jf.intel.com ([10.7.209.65]) by fmsmga103.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 15 May 2019 03:04:20 -0700 X-ExtLoop1: 1 Received: from um.fi.intel.com (HELO localhost) ([10.237.72.63]) by orsmga008.jf.intel.com with ESMTP; 15 May 2019 03:04:17 -0700 From: Alexander Shishkin To: Peter Zijlstra Cc: Yabin Cui , Ingo Molnar , Arnaldo Carvalho de Melo , Jiri Olsa , Namhyung Kim , linux-kernel@vger.kernel.org, alexander.shishkin@linux.intel.com Subject: Re: [PATCH] perf/ring_buffer: Fix exposing a temporarily decreased data_head. In-Reply-To: <20190515094352.GC2623@hirez.programming.kicks-ass.net> References: <20190515003059.23920-1-yabinc@google.com> <87ef50xlb8.fsf@ashishki-desk.ger.corp.intel.com> <20190515094352.GC2623@hirez.programming.kicks-ass.net> Date: Wed, 15 May 2019 13:04:16 +0300 Message-ID: <87bm04xcdb.fsf@ashishki-desk.ger.corp.intel.com> MIME-Version: 1.0 Content-Type: text/plain Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Peter Zijlstra writes: > On Wed, May 15, 2019 at 09:51:07AM +0300, Alexander Shishkin wrote: >> Yabin Cui writes: >> >> > diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c >> > index 674b35383491..0b9aefe13b04 100644 >> > --- a/kernel/events/ring_buffer.c >> > +++ b/kernel/events/ring_buffer.c >> > @@ -54,8 +54,10 @@ static void perf_output_put_handle(struct perf_output_handle *handle) >> > * IRQ/NMI can happen here, which means we can miss a head update. >> > */ >> > >> > - if (!local_dec_and_test(&rb->nest)) >> > + if (local_read(&rb->nest) > 1) { >> > + local_dec(&rb->nest); >> >> What stops rb->nest changing between local_read() and local_dec()? > > Nothing, however it must remain the same :-) > > That is the cryptic way of saying that since these buffers are strictly > per-cpu, the only change can come from interrupts, and they must have a > net 0 change. Or rather, an equal amount of decrements to increments. > > So if it changes, it must also change back to where it was. Ah that's true. So the whole ->nest thing can be done with READ_ONCE()/WRITE_ONCE() instead? Because the use of local_dec_and_test() creates an impression that we rely on atomicity of it, which in actuality we don't. Regards, -- Alex