Re: [PATCH 3/5] blktrace: refcount the request_queue during ioctl

From: Bart Van Assche <bvanassche@acm.org>
To: Luis Chamberlain <mcgrof@kernel.org>,
	Christoph Hellwig <hch@infradead.org>
Cc: axboe@kernel.dk, viro@zeniv.linux.org.uk,
	gregkh@linuxfoundation.org, rostedt@goodmis.org,
	mingo@redhat.com, jack@suse.cz, ming.lei@redhat.com,
	nstange@suse.de, akpm@linux-foundation.org, mhocko@suse.com,
	yukuai3@huawei.com, linux-block@vger.kernel.org,
	linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org, Omar Sandoval <osandov@fb.com>,
	Hannes Reinecke <hare@suse.com>, Michal Hocko <mhocko@kernel.org>
Subject: Re: [PATCH 3/5] blktrace: refcount the request_queue during ioctl
Date: Wed, 15 Apr 2020 07:18:22 -0700	[thread overview]
Message-ID: <73332d32-b095-507f-fb2a-68460533eeb7@acm.org> (raw)
In-Reply-To: <20200415123434.GU11244@42.do-not-panic.com>

On 2020-04-15 05:34, Luis Chamberlain wrote:
> On Wed, Apr 15, 2020 at 12:14:25AM -0700, Christoph Hellwig wrote:
>> 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.

blk_get_queue() prevents concurrent freeing of struct request_queue but
does not prevent concurrent blk_cleanup_queue() calls. Callers of
blk_get_queue() may encounter a change of the queue state from normal
into dying any time during the blk_get_queue() call or after
blk_get_queue() has finished. Maybe I'm overlooking something but I
doubt that modifying blk_get_queue() will help.

Bart.