linux-kernel - Re: [PATCH 3/5] blktrace: refcount the request

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Message-ID: <20200415071425.GA21099@infradead.org>
Date:   Wed, 15 Apr 2020 00:14:25 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Luis Chamberlain <mcgrof@...nel.org>
Cc:     Christoph Hellwig <hch@...radead.org>, axboe@...nel.dk,
        viro@...iv.linux.org.uk, bvanassche@....org,
        gregkh@...uxfoundation.org, rostedt@...dmis.org, mingo@...hat.com,
        jack@...e.cz, ming.lei@...hat.com, nstange@...e.de,
        akpm@...ux-foundation.org, mhocko@...e.com, yukuai3@...wei.com,
        linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Omar Sandoval <osandov@...com>,
        Hannes Reinecke <hare@...e.com>,
        Michal Hocko <mhocko@...nel.org>
Subject: Re: [PATCH 3/5] blktrace: refcount the request_queue during ioctl

On Wed, Apr 15, 2020 at 06:16:49AM +0000, Luis Chamberlain wrote:
> The BLKTRACESETUP above works on request_queue which later
> LOOP_CTL_DEL races on and sweeps the debugfs dir underneath us.
> If you use this commit alone though, this doesn't fix the race issue
> however, and that's because of both still the debugfs_lookup() use
> and that we're still using asynchronous removal at this point.
> 
> refcounting will just ensure we don't take the request_queue underneath
> our noses.
> 
> Should I just add this to the commit log?

That sounds much more useful than the trace.

Btw, Isn't blk_get_queue racy as well?  Shouldn't we check
blk_queue_dying after getting the reference and undo it if the queue is
indeeed dying?