scheduler hang on cpu re-hotplug with 2.6.27rc8

[Rafael, something for the regression list]

While testing cpu hotunplug/hotreplug (first 
setting two CPUs to offline and then to online again) on a 16 thread machine
with 2.6.27rc8 the first 

# echo 1 > ./devices/system/cpu/cpu14/online

after hotunplug deadlocked somewhere in the scheduler:

bash          D 00000000ffffcb5b     0  4683   4671
 ffff8804bc583c68 0000000000000086 ffff8804bc9d8640 0000000000000296
 ffff8804bdd34730 ffff8804be6fc090 ffff8804bdd34978 0000000c805a1e2a
 ffff8804be4fd780 ffffffff802298b4 ffffffff808acd98 ffff88027d0b1168
Call Trace:
 [<ffffffff802298b4>] __dequeue_entity+0x25/0x68
 [<ffffffff805a1b4b>] schedule_timeout+0x1e/0xad
 [<ffffffff8022a11f>] __disable_runtime+0x57/0x155
 [<ffffffff8025cc47>] cpupri_set+0xbe/0xcd
 [<ffffffff805a19b3>] wait_for_common+0xcd/0x131
 [<ffffffff8022c918>] default_wake_function+0x0/0xe
 [<ffffffff80241f3a>] synchronize_rcu+0x30/0x36
 [<ffffffff80241fac>] wakeme_after_rcu+0x0/0xc
 [<ffffffff8022dab6>] partition_sched_domains+0x9b/0x1dd
 [<ffffffff8022dc26>] update_sched_domains+0x2e/0x35
 [<ffffffff805a5297>] notifier_call_chain+0x29/0x4c
 [<ffffffff8059ef76>] _cpu_up+0xd0/0x10a
 [<ffffffff8059f004>] cpu_up+0x54/0x61
 [<ffffffff805837d5>] store_online+0x43/0x67
 [<ffffffff802c51e9>] sysfs_write_file+0xd2/0x110
 [<ffffffff8028656b>] vfs_write+0xad/0x136
 [<ffffffff802869f3>] sys_write+0x45/0x6e
 [<ffffffff8020b22b>] system_call_fastpath+0x16/0x1b

It just hung forever, but the machine was otherwise fully functional.

This was without frame pointers so the backtrace presumably has
some garbage. Haven't looked too closely.

-Andi

--

-- 
ak <at> linux.intel.com