[2.6.33-rc5] Weird deadlock when shutting down

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

Hi Ingo !

Johannes and I see this on our quad G5s... it -could- be similar to
one reported a short while ago by Xiaotian Feng <xtfeng@gmail.com>
under the subject [2.6.33-rc4] sysfs lockdep warnings on cpu hotplug.
 
Basically, the machine deadlocks right after printing the following
when doing a shutdown:

halt/4071 is trying to acquire lock:
 (s_active){++++.+}, at: [<c0000000001ef868>] .sysfs_addrm_finish+0x58/0xc0

but task is already holding lock:
 (&per_cpu(cpu_policy_rwsem, cpu)){+.+.+.}, at: [<c0000000004cd6ac>] .lock_policy_rwsem_write+0x84/0xf4

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

<nothing else ... machine deadlocked here>

Any idea ?

Cheers,
Ben.