From: Bart Van Assche <Bart.VanAssche@sandisk.com>
To: "tom.leiming@gmail.com" <tom.leiming@gmail.com>
CC: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "hch@infradead.org" <hch@infradead.org>,
        "linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
        "yizhan@redhat.com" <yizhan@redhat.com>, "axboe@fb.com" <axboe@fb.com>,
        "tj@kernel.org" <tj@kernel.org>
Subject: Re: [PATCH v1 3/3] blk-mq: start to freeze queue just after setting
 dying
Thread-Topic: [PATCH v1 3/3] blk-mq: start to freeze queue just after
 setting dying
Thread-Index: AQHSn145Mk+PEYhJp0SjoHMAamSd2g==
Date: Fri, 17 Mar 2017 20:36:58 +0000
Message-ID: <1489783004.2826.13.camel@sandisk.com>
References: <20170317095711.5819-1-tom.leiming@gmail.com>
         <20170317095711.5819-4-tom.leiming@gmail.com>
         <1489771915.2826.4.camel@sandisk.com>
         <CACVXFVPF0Sx5HyU+xw0MyghYMWifZ0SDH0YCtM-jrhieOJOJGg@mail.gmail.com>
In-Reply-To: <CACVXFVPF0Sx5HyU+xw0MyghYMWifZ0SDH0YCtM-jrhieOJOJGg@mail.gmail.com>
Accept-Language: en-US, nl-NL
Content-Language: en-US
Content-Type: text/plain; charset="iso-8859-1"
Content-ID: <79C798E6482AA148ACD4533E19EC7525@sandisk.com>
MIME-Version: 1.0
WDCIPOUTBOUND: EOP-TRUE
SpamDiagnosticOutput: 1:99
SpamDiagnosticMetadata: NSPM
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 17 Mar 2017 20:37:02.2124
 (UTC)
X-MS-Exchange-CrossTenant-Id: b61c8803-16f3-4c35-9b17-6f65f441df86
X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=b61c8803-16f3-4c35-9b17-6f65f441df86;Ip=[74.221.232.54];Helo=[sacsmgep14.sandisk.com]
X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem
X-MS-Exchange-Transport-CrossTenantHeadersStamped: CY4PR04MB0742
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org
Content-Transfer-Encoding: 8bit
X-MIME-Autoconverted: from quoted-printable to 8bit by mail.home.local id v2HKdgZU004310
Content-Length: 619
Lines: 20

On Sat, 2017-03-18 at 02:32 +0800, Ming Lei wrote:
> On Sat, Mar 18, 2017 at 1:32 AM, Bart Van Assche wrote:
> > +       /*
> > +        * Avoid that the updates of the queue flags and q_usage_counter
> > +        * are reordered.
> > +        */
> > +       smp_wmb();
> 
> atomic_inc_return() in blk_mq_freeze_queue_start() does imply a
> barrier(smp_mb()).

Hello Ming,

It's probably a good idea to mention that in a comment. The implementation
of blk_mq_freeze_queue_start() namely could be changed in the future such
that it uses another atomic operation that doesn't implicitly perform smp_mb().

Thanks,

Bart.