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=-10.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 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 9E356C433DB for ; Wed, 3 Feb 2021 13:29:57 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 4C02764F55 for ; Wed, 3 Feb 2021 13:29:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230123AbhBCN3h (ORCPT ); Wed, 3 Feb 2021 08:29:37 -0500 Received: from mx2.suse.de ([195.135.220.15]:56214 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230380AbhBCN3d (ORCPT ); Wed, 3 Feb 2021 08:29:33 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id B2399B12F; Wed, 3 Feb 2021 13:28:51 +0000 (UTC) Received: by ds.suse.cz (Postfix, from userid 10065) id 52DE0DA6FC; Wed, 3 Feb 2021 14:27:01 +0100 (CET) Date: Wed, 3 Feb 2021 14:27:01 +0100 From: David Sterba To: Qu Wenruo Cc: linux-btrfs@vger.kernel.org, Dan Carpenter Subject: Re: [PATCH] btrfs: add comment on why we can return 0 if we failed to atomically lock the page in read_extent_buffer_pages() Message-ID: <20210203132701.GD1993@twin.jikos.cz> Reply-To: dsterba@suse.cz Mail-Followup-To: dsterba@suse.cz, Qu Wenruo , linux-btrfs@vger.kernel.org, Dan Carpenter References: <20210128112508.123614-1-wqu@suse.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210128112508.123614-1-wqu@suse.com> User-Agent: Mutt/1.5.23.1-rc1 (2014-03-12) Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org On Thu, Jan 28, 2021 at 07:25:08PM +0800, Qu Wenruo wrote: > In read_extent_buffer_pages(), if we failed to lock the page atomically, > we just exit with return value 0. > > This is pretty counter-intuitive, as normally if we can't lock what we > need, we would return something like -EAGAIN. > > But the that return hides under (wait == WAIT_NONE) branch, which only > get triggered for readahead. > > And for readahead, if we failed to lock the page, it means the extent > buffer is either being read by other thread, or has been read and is > under modification. > Either way the eb will or has been cached, thus readahead has no need to > wait for it. > > This patch will add extra comment on this counter-intuitive behavior. > > Reported-by: Dan Carpenter > Signed-off-by: Qu Wenruo Added to misc-next, thanks, I've slightly rephrased the subject.