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

From: Luis Chamberlain <mcgrof@kernel.org>
To: Ming Lei <ming.lei@redhat.com>
Cc: axboe@kernel.dk, viro@zeniv.linux.org.uk, bvanassche@acm.org,
	gregkh@linuxfoundation.org, rostedt@goodmis.org,
	mingo@redhat.com, jack@suse.cz, 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: Thu, 16 Apr 2020 05:36:22 +0000	[thread overview]
Message-ID: <20200416053622.GJ11244@42.do-not-panic.com> (raw)
In-Reply-To: <20200416023122.GB2717677@T590>

On Thu, Apr 16, 2020 at 10:31:22AM +0800, Ming Lei wrote:
> On Tue, Apr 14, 2020 at 04:19:00AM +0000, Luis Chamberlain wrote:
> > Ensure that the request_queue is refcounted during its full
> > ioctl cycle. This avoids possible races against removal, given
> > blk_get_queue() also checks to ensure the queue is not dying.
> > 
> > This small race is possible if you defer removal of the request_queue
> > and userspace fires off an ioctl for the device in the meantime.
> > 
> > Cc: Bart Van Assche <bvanassche@acm.org>
> > Cc: Omar Sandoval <osandov@fb.com>
> > Cc: Hannes Reinecke <hare@suse.com>
> > Cc: Nicolai Stange <nstange@suse.de>
> > Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > Cc: Michal Hocko <mhocko@kernel.org>
> > Cc: yu kuai <yukuai3@huawei.com>
> > Reviewed-by: Bart Van Assche <bvanassche@acm.org>
> > Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
> > ---
> >  kernel/trace/blktrace.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> > 
> > diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c
> > index 15086227592f..17e144d15779 100644
> > --- a/kernel/trace/blktrace.c
> > +++ b/kernel/trace/blktrace.c
> > @@ -701,6 +701,9 @@ int blk_trace_ioctl(struct block_device *bdev, unsigned cmd, char __user *arg)
> >  	if (!q)
> >  		return -ENXIO;
> >  
> > +	if (!blk_get_queue(q))
> > +		return -ENXIO;
> > +
> >  	mutex_lock(&q->blk_trace_mutex);
> >  
> >  	switch (cmd) {
> > @@ -729,6 +732,9 @@ int blk_trace_ioctl(struct block_device *bdev, unsigned cmd, char __user *arg)
> >  	}
> >  
> >  	mutex_unlock(&q->blk_trace_mutex);
> > +
> > +	blk_put_queue(q);
> > +
> >  	return ret;
> >  }
> 
> Actually when bdev is opened, one extra refcount is held on gendisk, so
> gendisk won't go away. And __device_add_disk() does grab one extra
> refcount on request queue, so request queue shouldn't go away when ioctl
> is running.

Alright, then yes, this should not be needed.

  Luis