On Sat, Jan 30, 2010 at 12:44:51PM +0100, Pawel Jakub Dawidek wrote: > On Sat, Jan 30, 2010 at 12:27:49PM +0100, Pawel Jakub Dawidek wrote: > > On Sat, Jan 30, 2010 at 12:58:26AM +0200, Alexander Motin wrote: > > > Hi. > > > > > > Experimenting with SATA hot-plug I've found quite repeatable deadlock > > > case. Problem observed when several SATA devices, opened via devfs, > > > disappear at exactly same time. In my case, at time of unplugging SATA > > > Port Multiplier with several disks beyond it. All I have to do is to run > > > several `dd if=/dev/adaX of=/dev/null bs=1m &` commands and unplug > > > multiplier. That causes predictable I/O errors and devices destruction. > > > But with high probability several dd processes getting stuck in kernel. > > [...] > > > > I observed the same thing yesterday while stress-testing HAST: > > > > 3659 2504 3659 0 DE+ GEOM top 0x8079a348 dd > > 3658 2102 2102 0 DE+ GEOM top 0x8079a348 hastd > > 2 0 0 0 DL devdrn 0x85b1bc68 [g_event] > > > > Both dd(1) and hastd(8) wait for the GEOM topology lock in the exit path, > > which is already held by the g_event thread. > > Maybe I'll add how I understand what's going on: > > GEOM calls destroy_dev() while holding the topology lock. > > Destroy_dev() wants to destroy device, but can't because there are > threads that still have it open. > > The threads can't close it, because to close it they need the topology > lock. > > The deadlock is quite obvious, IMHO. > > I believe the problem could be solved by dropping the topology lock in > g_dev_orphan() when calling destroy_dev(dev), but it is hard to say if > it is safe to drop the topology lock there. Maybe Poul-Henning could > take a look.
As I already said, if you cannot drop a lock, destroy_dev_sched() is designed to handle this. You should be careful to not allow any further activitity on the device scheduled for destruction.
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