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=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED,USER_AGENT_MUTT 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 3BA0AC32789 for ; Tue, 6 Nov 2018 11:54:41 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 0DD2420862 for ; Tue, 6 Nov 2018 11:54:41 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 0DD2420862 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com 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 S1730728AbeKFVTa (ORCPT ); Tue, 6 Nov 2018 16:19:30 -0500 Received: from mx1.redhat.com ([209.132.183.28]:41880 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730271AbeKFVT3 (ORCPT ); Tue, 6 Nov 2018 16:19:29 -0500 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 5ADDC305B878; Tue, 6 Nov 2018 11:54:39 +0000 (UTC) Received: from krava (unknown [10.40.205.136]) by smtp.corp.redhat.com (Postfix) with SMTP id 45B68600C0; Tue, 6 Nov 2018 11:54:36 +0000 (UTC) Date: Tue, 6 Nov 2018 12:54:36 +0100 From: Jiri Olsa To: David Miller Cc: acme@kernel.org, linux-kernel@vger.kernel.org, namhyung@kernel.org, jolsa@kernel.org Subject: Re: [PATCH RFC] hist lookups Message-ID: <20181106115436.GE3164@krava> References: <20181105203447.GA25674@krava> <20181105.194542.476367332910923203.davem@davemloft.net> <20181105.200321.1569039579935458312.davem@davemloft.net> <20181105.205342.1568518551892125916.davem@davemloft.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181105.205342.1568518551892125916.davem@davemloft.net> User-Agent: Mutt/1.10.1 (2018-07-13) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.41]); Tue, 06 Nov 2018 11:54:39 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Nov 05, 2018 at 08:53:42PM -0800, David Miller wrote: > > 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. ah right.. I had originaly full mutex around that, then I switched it off in the last patch and did not realize this implication.. nice ;-) does it fix the crash you reported earlier? thanks, jirka