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

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Date:   Wed, 15 Apr 2020 12:34:34 +0000
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     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 12:14:25AM -0700, Christoph Hellwig wrote:
> 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?

Yes that race should be possible:

bool blk_get_queue(struct request_queue *q)                                     
{                                                                               
	if (likely(!blk_queue_dying(q))) {
       ----------> we can get the queue to go dying here <---------
		__blk_get_queue(q);
		return true;
	}                                                                       

	return false;
}                                                                               
EXPORT_SYMBOL(blk_get_queue);

I'll pile up a fix. I've also considered doing a full review of callers
outside of the core block layer using it, and maybe just unexporting
this. It was originally exported due to commit d86e0e83b ("block: export
blk_{get,put}_queue()") to fix a scsi bug, but I can't find such
respective fix. I suspec that using bdgrab()/bdput() seems more likely
what drivers should be using. That would allow us to keep this
functionality internal.

Think that's worthy review?

  Luis