Messages in this thread | | | Subject | Re: 2.6.17-rc6-mm1/pktcdvd - BUG: possible circular locking | From | Peter Osterlund <> | Date | 14 Jul 2006 13:22:57 +0200 |
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Arjan van de Ven <arjan@linux.intel.com> writes:
> On Thu, 2006-06-22 at 16:50 +0200, Peter Osterlund wrote: > > 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. > > and what locking prevents this? And via multiple opens?
You are right that my reasoning was incorrect. If someone is doing "pktsetup ; pktsetup -d" quickly in a loop while someone else is trying to open the device, one thread could be at the start of pkt_open() at the same time as another thread is in pkt_new_dev().
However, I added a 5s delay in pkt_open() to enlarge the race window. I still couldn't make the driver lock up though. The explanation is that pkt_new_dev() calls blkdev_get() with the CD device (eg /dev/hdc) as bdev parameter, while do_open() locks the bd_mutex for the pktcdvd device (eg /dev/pktcdvd/0).
Do you still think this could deadlock? If not, how should the code be annotated to make this warning go away?
-- Peter Osterlund - petero2@telia.com http://web.telia.com/~u89404340 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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