Re: [PATCH 2/7] block: loop: don't hold lo_ctl_mutex in lo_open

Jarod Wilson
2015-04-08T15:28:57Z

On Wed, Apr 08, 2015 at 10:20:22PM +0800, Ming Lei wrote:
> On Wed, Apr 8, 2015 at 10:00 PM, Jarod Wilson <jarod@redhat.com> wrote:
> > On Wed, Apr 08, 2015 at 09:40:34AM -0400, Jarod Wilson wrote:
> >> On Wed, Apr 08, 2015 at 02:50:59PM +0800, Ming Lei wrote:
> >> > Hi Jarod,
> >> >
> >> > On Wed, Apr 8, 2015 at 2:23 PM, Jarod Wilson <jarod@redhat.com> wrote:
> >> > > From: Ming Lei <ming.lei@canonical.com>
> >> > >
> >> > > The lo_ctl_mutex is held for running all ioctl handlers, and
> >> > > in some ioctl handlers, ioctl_by_bdev(BLKRRPART) is called for
> >> > > rereading partitions, which requires bd_mutex.
> >> > >
> >> > > So it is easy to cause failure because trylock(bd_mutex) may
> >> > > fail inside blkdev_reread_part(), and follows the lock context:
> >> > >
> >> > > blkid or other application:
> >> > > ->open()
> >> > > ->mutex_lock(bd_mutex)
> >> > > ->lo_open()
> >> > > ->mutex_lock(lo_ctl_mutex)
> >> > >
> >> > > losetup(set fd ioctl):
> >> > > ->mutex_lock(lo_ctl_mutex)
> >> > > ->ioctl_by_bdev(BLKRRPART)
> >> > > ->trylock(bd_mutex)
> >> > >
> >> > > This patch trys to eliminate the ABBA lock dependency by removing
> >> > > lo_ctl_mutext in lo_open() with the following approach:
> >> > >
> >> > > 1) introduce lo_open_mutex to protect lo_refcnt and avoid acquiring
> >> > > lo_ctl_mutex in lo_open():
> >> >
> >> > It is a bit quick since I said the lo_open_mutex can be removed,
> >> > and Christoph agreed that too.
> >> >
> >> > So looks we still need to post another version, :-)
> >>
> >> Ah. I missed that bit. Just trying to keep up momentum.
> >
> > Are you working on that atomic_t version then, or should I dive into it?
>
> It has been ready, and I will post them out once it passes my tests.

Okay, I'll hold tight then. Sorry for the noise. :p

--
Jarod Wilson
jarod@redhat.com