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

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Message-ID: <20200416053622.GJ11244@42.do-not-panic.com>
Date:   Thu, 16 Apr 2020 05:36:22 +0000
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     axboe@...nel.dk, viro@...iv.linux.org.uk, bvanassche@....org,
        gregkh@...uxfoundation.org, rostedt@...dmis.org, mingo@...hat.com,
        jack@...e.cz, 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 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@....org>
> > Cc: Omar Sandoval <osandov@...com>
> > Cc: Hannes Reinecke <hare@...e.com>
> > Cc: Nicolai Stange <nstange@...e.de>
> > Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > Cc: Michal Hocko <mhocko@...nel.org>
> > Cc: yu kuai <yukuai3@...wei.com>
> > Reviewed-by: Bart Van Assche <bvanassche@....org>
> > Signed-off-by: Luis Chamberlain <mcgrof@...nel.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