linux-kernel - Re: [PATCH v4] blktrace: Fix potentail deadlock between delete & sysfs ops

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Date:   Fri, 18 Aug 2017 20:18:13 +0000
From:   Bart Van Assche <Bart.VanAssche@....com>
To:     "mingo@...nel.org" <mingo@...nel.org>,
        "longman@...hat.com" <longman@...hat.com>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "axboe@...nel.dk" <axboe@...nel.dk>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>
Subject: Re: [PATCH v4] blktrace: Fix potentail deadlock between delete &
 sysfs ops

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.