From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on dcvr.yhbt.net X-Spam-Level: X-Spam-ASN: AS31976 209.132.180.0/23 X-Spam-Status: No, score=-4.0 required=3.0 tests=AWL,BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,RCVD_IN_DNSWL_HI shortcircuit=no autolearn=ham autolearn_force=no version=3.4.2 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by dcvr.yhbt.net (Postfix) with ESMTP id ACAED20248 for ; Thu, 14 Mar 2019 00:45:28 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726512AbfCNAp1 (ORCPT ); Wed, 13 Mar 2019 20:45:27 -0400 Received: from cloud.peff.net ([104.130.231.41]:50134 "HELO cloud.peff.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1726412AbfCNAp1 (ORCPT ); Wed, 13 Mar 2019 20:45:27 -0400 Received: (qmail 31861 invoked by uid 109); 14 Mar 2019 00:45:28 -0000 Received: from Unknown (HELO peff.net) (10.0.1.2) by cloud.peff.net (qpsmtpd/0.94) with SMTP; Thu, 14 Mar 2019 00:45:28 +0000 Authentication-Results: cloud.peff.net; auth=none Received: (qmail 17930 invoked by uid 111); 14 Mar 2019 00:44:31 -0000 Received: from sigill.intra.peff.net (HELO sigill.intra.peff.net) (10.0.0.7) by peff.net (qpsmtpd/0.94) with (ECDHE-RSA-AES256-GCM-SHA384 encrypted) SMTP; Wed, 13 Mar 2019 20:44:31 -0400 Authentication-Results: peff.net; auth=none Received: by sigill.intra.peff.net (sSMTP sendmail emulation); Wed, 13 Mar 2019 20:44:10 -0400 Date: Wed, 13 Mar 2019 20:44:10 -0400 From: Jeff King To: =?utf-8?B?w4Z2YXIgQXJuZmrDtnLDsA==?= Bjarmason Cc: git@vger.kernel.org, Junio C Hamano , =?utf-8?B?Tmd1eeG7hW4gVGjDoWkgTmfhu41j?= Duy , Michael Haggerty , Stefan Beller , Jonathan Nieder Subject: Re: [PATCH 5/5] reflog expire: don't assert the OID when locking refs Message-ID: <20190314004409.GJ31968@sigill.intra.peff.net> References: <87imwmbv7l.fsf@evledraar.gmail.com> <20190313235439.30439-6-avarab@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20190313235439.30439-6-avarab@gmail.com> Sender: git-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: git@vger.kernel.org On Thu, Mar 14, 2019 at 12:54:39AM +0100, Ævar Arnfjörð Bjarmason wrote: > The locking protocol for reflogs involves getting a *.lock file on the > loose ref[1] (even if the actual ref is packed). This isn't needed to > expire the reflog, and needlessly results promotes reference update > contention to hard errors in e.g. "gc". This first paragraph threw me off. It sounds like you are saying we don't need to take a lock, but we absolutely do. It's just that we don't need to care about the lock having some particular value. Which you do go on to explain, but I think it would be more clear by simply removing this first paragraph. > If we instead lock the reference without considering what OID we last > saw it at, we won't encounter user-visible contention to the extent > that core.filesRefLockTimeout mitigates it. See 4ff0f01cb7 ("refs: > retry acquiring reference locks for 100ms", 2017-08-21). I think this part is true. I'd love to get a confirmation from Michael Haggerty, who has spent way more time thinking about ref and reflog locking than any mortal should have to. Hopefully he even still remembers some of it. :) -Peff