From: Bart Van Assche <Bart.VanAssche@wdc.com>
To: "mingo@kernel.org" <mingo@kernel.org>,
        "longman@redhat.com" <longman@redhat.com>,
        "rostedt@goodmis.org" <rostedt@goodmis.org>,
        "axboe@kernel.dk" <axboe@kernel.dk>
CC: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-block@vger.kernel.org" <linux-block@vger.kernel.org>
Subject: Re: [PATCH v4] blktrace: Fix potentail deadlock between delete &
 sysfs ops
Thread-Topic: [PATCH v4] blktrace: Fix potentail deadlock between delete &
 sysfs ops
Thread-Index: AQHTGFzHomVm/w0FJUW3QWQDXLjARqKKjYEA
Date: Fri, 18 Aug 2017 20:18:13 +0000
Message-ID: <1503087492.2622.13.camel@wdc.com>
References: <1503086474-20187-1-git-send-email-longman@redhat.com>
In-Reply-To: <1503086474-20187-1-git-send-email-longman@redhat.com>
Accept-Language: en-US
Content-Language: en-US
wdcipoutbound: EOP-TRUE
spamdiagnosticoutput: 1:99
spamdiagnosticmetadata: NSPM
Content-Type: text/plain; charset="utf-8"
Content-ID: <A5BC9BDF3ABAF2458C365EEE89896840@namprd04.prod.outlook.com>
MIME-Version: 1.0
X-MS-Exchange-CrossTenant-originalarrivaltime: 18 Aug 2017 20:18:13.7199
 (UTC)
X-MS-Exchange-CrossTenant-fromentityheader: Hosted
X-MS-Exchange-CrossTenant-id: b61c8803-16f3-4c35-9b17-6f65f441df86
X-MS-Exchange-Transport-CrossTenantHeadersStamped: CY1PR0401MB0908
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 base64 to 8bit by nfs id v7IKJGRl027032
Content-Length: 1368
Lines: 32

On Fri, 2017-08-18 at 16:01 -0400, Waiman Long wrote:
> [ ... ]
> Instead of using bd_mutex, a new global blktrace mutex is now used
> to protect against concurrent access, creation and destruction of the
> blk_trace structure that is used only in the blktrace.c file. As
> blktrace files will not be frequently accessed, using a global mutex
> should not cause any performance problem.
> [ ... ]
> +/*
> + * The bd_mutex was used previously for protecting blk_trace structure.
> + * That could lead to deadlock with concurrent block device deletion and
> + * sysfs access. So a global blktrace_mutex is now used instead for
> + * protecting the blk_trace structure.
> + *
> + * The references to the opened sysfs or device files should prevent the
> + * underlying block device from being removed.
> + */
> +static DEFINE_MUTEX(blktrace_mutex);

Hello Waiman,

Thanks for having addressed my previous comment. Regarding this patch: sorry
but I don't think it's a good idea to use a global mutex for serializing
accesses to tracing data of a single block device. Global mutexes create
unwanted lock dependencies between different block devices. Additionally, on
multiprocessor systems global mutexes can cause cache line ping-pong between
processors and hence can cause a severe slowdown. Please make blktrace_mutex
per block device instead of global.

Thanks,

Bart.