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@xxxxxxxxxx> 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@xxxxxxxxxxxxxxx # 4.14+
> > > Reported-by: Bart Van Assche <Bart.VanAssche@xxxxxxx>
> > > Signed-off-by: Mike Snitzer <snitzer@xxxxxxxxxx>
> > > ---
> > > 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.
Thanks,
Ming
