Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751824AbdIMDyN (ORCPT ); Tue, 12 Sep 2017 23:54:13 -0400 Received: from mail-pg0-f44.google.com ([74.125.83.44]:34286 "EHLO mail-pg0-f44.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751381AbdIMDyK (ORCPT ); Tue, 12 Sep 2017 23:54:10 -0400 X-Google-Smtp-Source: ADKCNb4HHAerRX90c1gXWh0L9U2c+qalTHCNpJzRpZ41rKGmnQc//9RD0xdi//xE2Nnf/qt29nsHHw== Subject: Re: [PATCH] blk-mq: put the driver tag of nxt rq before first one is requeued To: "jianchao.wang" , Ming Lei Cc: linux-block@vger.kernel.org, linux-kernel@vger.kernel.org References: <1505236475-9209-1-git-send-email-jianchao.w.wang@oracle.com> <20170912102302.GC15792@ming.t460p> <23b700aa-ed37-1081-d980-4073804f293b@oracle.com> <20170913012452.GA24642@ming.t460p> <4b7dcb10-9ec5-9a00-8d16-009e83ef7b5b@oracle.com> <96a21c69-cf79-58a2-2c71-2e440abf9b36@oracle.com> <6327de01-0bf4-7e79-59ad-f6489378bfd5@kernel.dk> From: Jens Axboe Message-ID: <17775b50-1bf2-26d2-c371-2eb349be85d6@kernel.dk> Date: Tue, 12 Sep 2017 21:54:06 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1044 Lines: 27 On 09/12/2017 09:39 PM, jianchao.wang wrote: >> Exactly, and especially the readability is the key element here. It's >> just not worth it to try and be too clever, especially not for >> something like this. When you read the above, you immediately know >> what the code does without needing a comment. That's not true for the >> other construct. You both have to read other parts of the function >> to figure out what it does, AND read the entire function to ensure it >> always does the right thing. Fragile. > > Thanks for your comments , jens and ming. I＇m really appreciative of > that. About the fragility, do you mean the possibility that may > release the tag of the next rq which has a driver tag itself (maybe a > flush) ? I mean that if you do: if (!list_empty(list)) blk_mq_put_driver_tag(nxt); it's fragile code in the sense that changes elsewhere in the function are harder to validate and/or can impact the functionality of that simple if and tag put. The actual release must always be safe, of course. -- Jens Axboe