Re: [for-4.16 PATCH v4 2/4] block: use queue_lock when clearing QUEUE_FLAG_REGISTERED in blk_unregister_queue

[Mike Snitzer <snitzer@redhat.com>]

On Fri, Jan 12 2018 at  9:14am -0500,
Ming Lei <ming.lei@redhat.com> wrote:

> On Fri, Jan 12, 2018 at 07:53:40AM -0500, Mike Snitzer wrote:
> > On Fri, Jan 12 2018 at  2:09am -0500,
> > Ming Lei <ming.lei@redhat.com> wrote:
> > 
> > > On Thu, Jan 11, 2018 at 03:14:15PM -0500, Mike Snitzer wrote:
> > > > blk_unregister_queue() must protect against any modifications of
> > > > q->queue_flags (not just those performed in blk-sysfs.c).  Therefore
> > > > q->queue_lock needs to be used rather than q->sysfs_lock.
> > > > 
> > > > Fixes: e9a823fb34a8b ("block: fix warning when I/O elevator is changed as request_queue is being removed")
> > > > Cc: stable@vger.kernel.org # 4.14+
> > > > Reported-by: Bart Van Assche <Bart.VanAssche@wdc.com>
> > > > Signed-off-by: Mike Snitzer <snitzer@redhat.com>
> > > > ---
> > > >  block/blk-sysfs.c | 4 ++--
> > > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > > > 
> > > > diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
> > > > index 870484eaed1f..52f57539f1c7 100644
> > > > --- a/block/blk-sysfs.c
> > > > +++ b/block/blk-sysfs.c
> > > > @@ -929,9 +929,9 @@ void blk_unregister_queue(struct gendisk *disk)
> > > >  	if (WARN_ON(!q))
> > > >  		return;
> > > >  
> > > > -	mutex_lock(&q->sysfs_lock);
> > > > +	spin_lock_irq(q->queue_lock);
> > > >  	queue_flag_clear_unlocked(QUEUE_FLAG_REGISTERED, q);
> > > > -	mutex_unlock(&q->sysfs_lock);
> > > > +	spin_unlock_irq(q->queue_lock);
> > > >  
> > > >  	wbt_exit(q);
> > > 
> > > Hi Mike,
> > > 
> > > This change may not be correct, since at least e9a823fb34a8b depends
> > > on q->sysfs_lock to sync between testing the flag in __elevator_change()
> > > and clearing it here.
> > 
> > The header for commit e9a823fb34a8b says:
> >     To fix this warning, we can check the QUEUE_FLAG_REGISTERED flag when
> >     changing the elevator and use the request_queue's sysfs_lock to
> >     serialize between clearing the flag and the elevator testing the flag.
> > 
> > The reality is sysfs_lock isn't needed to serialize between
> > blk_unregister_queue() clearing the flag and __elevator_change() testing
> > the flag.
> 
> Without holding sysfs_lock, __elevator_change() may just be called after
> q->kobj is deleted from blk_unregister_queue(), then the warning of
> 'WARNING: CPU: 3 PID: 14075 at lib/kobject.c:244 kobject_add_internal+0x103/0x2d0'
> can be triggered again.
> 
> BTW, __elevator_change() is always run with holding sysfs_lock.

Yes, I'm well aware.  Please see v5's PATCH 02/04 which is inbound now.

Thanks,
Mike