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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED 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 70E2AC32789 for ; Tue, 6 Nov 2018 04:53:52 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 435B12083D for ; Tue, 6 Nov 2018 04:53:52 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 435B12083D Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=davemloft.net Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729613AbeKFORE (ORCPT ); Tue, 6 Nov 2018 09:17:04 -0500 Received: from shards.monkeyblade.net ([23.128.96.9]:39830 "EHLO shards.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729021AbeKFORE (ORCPT ); Tue, 6 Nov 2018 09:17:04 -0500 Received: from localhost (unknown [IPv6:2601:601:9f80:35cd::cf9]) (using TLSv1 with cipher AES256-SHA (256/256 bits)) (Client did not present a certificate) (Authenticated sender: davem-davemloft) by shards.monkeyblade.net (Postfix) with ESMTPSA id 0325314A6B938; Mon, 5 Nov 2018 20:53:42 -0800 (PST) Date: Mon, 05 Nov 2018 20:53:42 -0800 (PST) Message-Id: <20181105.205342.1568518551892125916.davem@davemloft.net> To: jolsa@redhat.com Cc: acme@kernel.org, linux-kernel@vger.kernel.org, namhyung@kernel.org, jolsa@kernel.org Subject: Re: [PATCH RFC] hist lookups From: David Miller In-Reply-To: <20181105.200321.1569039579935458312.davem@davemloft.net> References: <20181105203447.GA25674@krava> <20181105.194542.476367332910923203.davem@davemloft.net> <20181105.200321.1569039579935458312.davem@davemloft.net> X-Mailer: Mew version 6.8 on Emacs 26.1 Mime-Version: 1.0 Content-Type: Text/Plain; charset=us-ascii Content-Transfer-Encoding: 7bit X-Greylist: Sender succeeded SMTP AUTH, not delayed by milter-greylist-4.5.12 (shards.monkeyblade.net [149.20.54.216]); Mon, 05 Nov 2018 20:53:43 -0800 (PST) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Jiri, Because you now run queued_events__queue() lockless with that condvar trick, it is possible for top->qe.in to be seen as one past the data[] array, this is because the rotate_queues() code goes: if (++top->qe.in > &top->qe.data[1]) top->qe.in = &top->qe.data[0]; So for a brief moment top->qe.in is out of range and thus perf_top__mmap_read_idx() can try to enqueue to top->qe.data[2] We can just do: if (top->qe.in == &top->qe.data[1]) top->qe.in = &top->qe.data[0]; else top->qe.in = &top->qe.data[1]; Or, make top->qe.in an index, and simply go: top->qe.in ^= 1; Either way will fix the bug.