Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751890AbdIMD7o (ORCPT ); Tue, 12 Sep 2017 23:59:44 -0400 Received: from userp1040.oracle.com ([156.151.31.81]:23980 "EHLO userp1040.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751800AbdIMD7h (ORCPT ); Tue, 12 Sep 2017 23:59:37 -0400 Subject: Re: [PATCH] blk-mq: put the driver tag of nxt rq before first one is requeued To: Jens Axboe , 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> <17775b50-1bf2-26d2-c371-2eb349be85d6@kernel.dk> From: "jianchao.wang" Message-ID: Date: Wed, 13 Sep 2017 11:59:19 +0800 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: <17775b50-1bf2-26d2-c371-2eb349be85d6@kernel.dk> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit X-Source-IP: userv0021.oracle.com [156.151.31.71] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1133 Lines: 30 On 09/13/2017 11:54 AM, Jens Axboe wrote: > 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. > Got it, thanks a lot.