linux-kernel - Re: [PATCH v7] blktrace: Fix potentail deadlock between delete & sysfs ops

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Message-ID: <76cc6fea-fd1a-04fb-b18b-04ea5d69dde9@redhat.com>
Date:   Wed, 20 Sep 2017 15:05:36 -0400
From:   Waiman Long <longman@...hat.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Jens Axboe <axboe@...nel.dk>, Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org,
        linux-nilfs@...r.kernel.org, cluster-devel@...hat.com,
        Bart Van Assche <Bart.VanAssche@....com>
Subject: Re: [PATCH v7] blktrace: Fix potentail deadlock between delete &
 sysfs ops

On 09/20/2017 01:35 PM, Christoph Hellwig wrote:
>> +/*
>> + * When reading or writing the blktrace sysfs files, the references to the
>> + * opened sysfs or device files should prevent the underlying block device
>> + * from being removed. So no further delete protection is really needed.
>> + *
>> + * Protection from multiple readers and writers accessing blktrace data
>> + * concurrently is still required. The bd_mutex was used for this purpose.
>> + * That could lead to deadlock with concurrent block device deletion and
>> + * sysfs access. As a result, a new blk_trace_mutex is now added to be
>> + * used solely by the blktrace code.
>> + */
> Comments about previous locking schemes really don't have a business
> in the code - those are remarks for the commit logs.  And in general
> please explain the locking scheme near the data that they proctect
> it, as locks should always protected data, not code and the comments
> should follow that.

It seems to be a general practice that we don't put detailed comments in
the header files. The comment was put above the function with the first
instance of the blk_trace_mutex. Yes, I agree that talking about the
past history may not be applicable here. I will keep that in mind in the
future.

Thanks,
Longman