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,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 58DC5C43441 for ; Mon, 19 Nov 2018 09:13:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 20A1220817 for ; Mon, 19 Nov 2018 09:13:03 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 20A1220817 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 S1727403AbeKSTgC (ORCPT ); Mon, 19 Nov 2018 14:36:02 -0500 Received: from mx1.redhat.com ([209.132.183.28]:34672 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726988AbeKSTgC (ORCPT ); Mon, 19 Nov 2018 14:36:02 -0500 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id C7B193084242; Mon, 19 Nov 2018 09:13:00 +0000 (UTC) Received: from krava (ovpn-204-39.brq.redhat.com [10.40.204.39]) by smtp.corp.redhat.com (Postfix) with SMTP id 456CB19490; Mon, 19 Nov 2018 09:12:58 +0000 (UTC) Date: Mon, 19 Nov 2018 10:12:57 +0100 From: Jiri Olsa To: Namhyung Kim Cc: David Miller , acme@kernel.org, linux-kernel@vger.kernel.org, jolsa@kernel.org, kernel-team@lge.com Subject: Re: [PATCH RFC] hist lookups Message-ID: <20181119091257.GA8967@krava> References: <20181105203447.GA25674@krava> <20181105.194542.476367332910923203.davem@davemloft.net> <20181105.200321.1569039579935458312.davem@davemloft.net> <20181105.205342.1568518551892125916.davem@davemloft.net> <20181106115436.GE3164@krava> <20181119052603.GA20153@sejong> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181119052603.GA20153@sejong> User-Agent: Mutt/1.10.1 (2018-07-13) X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.40]); Mon, 19 Nov 2018 09:13:01 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Nov 19, 2018 at 02:26:03PM +0900, Namhyung Kim wrote: > Hi Jirka > > Sorry for late! > > On Tue, Nov 06, 2018 at 12:54:36PM +0100, Jiri Olsa wrote: > > 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 ;-) > > I like the rotate_queues() using cond-variable. Have you tried to use > the same for hists->lock in hists__get_rotate_entries_in() too? > > Eventually it'd be nice to avoid locks when a single thread processes > all the events. yep, I thought we could use it there as well, but it could be more tricky because we use hists->lock for that, which is used on other places as well.. will check thanks, jirka