linux-kernel - Re: [PATCH v6 1/2] blktrace: Fix potentail deadlock between delete & sysfs ops

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Date:   Tue, 19 Sep 2017 07:38:11 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        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 v6 1/2] blktrace: Fix potentail deadlock between delete &
 sysfs ops

On Tue, Sep 19, 2017 at 08:49:12AM -0400, Waiman Long wrote:
> On 09/18/2017 08:01 PM, Christoph Hellwig wrote:
> > Taking a look at this it seems like using a lock in struct block_device
> > isn't the right thing to do anyway - all the action is on fields in
> > struct blk_trace, so having a lock inside that would make a lot more
> > sense.
> >
> > It would also help to document what exactly we're actually protecting.
> 
> I think I documented in the patch that the lock has to protect changes
> in the blktrace structure as well as the allocation and destruction of
> it. Because of that, it can't be put inside the blktrace structure. The
> original code use the bd_mutex of the block_device structure. I just
> change the code to use another bd_fsfreeze_mutex in the same structure.

Either way it has absolutely nothing to do with struct block_device,
given that struct blk_trace hangs off the request_queue.

Reusing a mutex just because it happens to live in a structure also
generally is a bad idea if the protected data is in no way related.