Re: 2.6.17-rc6-mm1/pktcdvd - BUG: possible circular locking

[Peter Osterlund <petero2@telia.com>]

Arjan van de Ven <arjan@linux.intel.com> writes:

> Laurent Riffard wrote:
> > Hello,
> > This BUG happened while pktcdvd service was starting. Basically, the
> > 2 following commands were issued:
> > - modprobe ptkcdvd
> > - pktsetup dvd /dev/dvd
> 
> This appears to be a real bug:
> 
> A normal pkt dvd block dev open takes the
> bdev_mutex in the regular block device open path, which takes
> ctl_mutex in the pkt_open function which gets called then from
> the block layer.
> 
> HOWEVER the IOCTL path does it the other way around:
> 
>                  mutex_lock(&ctl_mutex);
>                  ret = pkt_setup_dev(&ctrl_cmd);
>                  mutex_unlock(&ctl_mutex);
> 
> where pkt_setup_dev in term calls pkt_new_dev which
> calls blkdev_get(), which takes the bdev_mutex.
> 
> Looks very much like a AB-BA deadlock to me...

I don't understand how this could deadlock. If the device is already
setup, pkt_new_dev() returns before calling blkdev_get(). If the
device is not already setup, the block device doesn't exist yet so
there can not be another caller in the pkt_open() path.

-- 
Peter Osterlund - petero2@telia.com
http://web.telia.com/~u89404340