lockdep warnings: cpufreq ondemand gorvernor possibly circular locking

[KOSAKI Motohiro]

Hi

my box output following warnings.
it seems regression by commit 7ccc7608b836e58fbacf65ee4f8eefa288e86fac.

A: work -> do_dbs_timer()  -> cpu_policy_rwsem
B: store() -> cpu_policy_rwsem -> cpufreq_governor_dbs() -> work



=======================================================
nd cpu frequency[ INFO: possible circular locking dependency detected ]
 scaling: 2.6.30-rc4-mm1 #26
-------------------------------------------------------
K99cpuspeed/227488 is trying to acquire lock:
 (&(&dbs_info->work)->work){+.+...}, at: [<ffffffff81055bfd>]
__cancel_work_timer+0xde/0x227

but task is already holding lock:
 (dbs_mutex){+.+.+.}, at: [<ffffffffa0081af3>]
cpufreq_governor_dbs+0x241/0x2d2 [cpufreq_ondemand]

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #2 (dbs_mutex){+.+.+.}:
       [<ffffffff8106a15a>] __lock_acquire+0xa9d/0xc33
       [<ffffffff8106a3b1>] lock_acquire+0xc1/0xe5
       [<ffffffff81303aa5>] __mutex_lock_common+0x4d/0x34c
       [<ffffffff81303e5c>] mutex_lock_nested+0x3a/0x3f
       [<ffffffffa008193d>] cpufreq_governor_dbs+0x8b/0x2d2 [cpufreq_ondemand]	
       [<ffffffff812678bd>] __cpufreq_governor+0xa7/0xe4
       [<ffffffff81267acf>] __cpufreq_set_policy+0x19a/0x216
       [<ffffffff81268572>] store_scaling_governor+0x1ec/0x228
       [<ffffffff812691fb>] store+0x67/0x8c					
       [<ffffffff81129091>] sysfs_write_file+0xe9/0x11e				
       [<ffffffff810e64d8>] vfs_write+0xb0/0x10a
       [<ffffffff810e6600>] sys_write+0x4c/0x74
       [<ffffffff8100bc1b>] system_call_fastpath+0x16/0x1b
       [<ffffffffffffffff>] 0xffffffffffffffff

-> #1 (&per_cpu(cpu_policy_rwsem, cpu)){+++++.}:
       [<ffffffff8106a15a>] __lock_acquire+0xa9d/0xc33
       [<ffffffff8106a3b1>] lock_acquire+0xc1/0xe5
       [<ffffffff813040d8>] down_write+0x4d/0x81
       [<ffffffff81268ad3>] lock_policy_rwsem_write+0x4d/0x7d			
       [<ffffffffa0081692>] do_dbs_timer+0x64/0x284 [cpufreq_ondemand]
       [<ffffffff81055395>] worker_thread+0x205/0x318				
       [<ffffffff8105933f>] kthread+0x8d/0x95
       [<ffffffff8100ccfa>] child_rip+0xa/0x20
       [<ffffffffffffffff>] 0xffffffffffffffff

-> #0 (&(&dbs_info->work)->work){+.+...}:
       [<ffffffff8106a04e>] __lock_acquire+0x991/0xc33
       [<ffffffff8106a3b1>] lock_acquire+0xc1/0xe5
       [<ffffffff81055c36>] __cancel_work_timer+0x117/0x227
       [<ffffffff81055d58>] cancel_delayed_work_sync+0x12/0x14				
       [<ffffffffa0081b06>] cpufreq_governor_dbs+0x254/0x2d2
[cpufreq_ondemand]
       [<ffffffff812678bd>] __cpufreq_governor+0xa7/0xe4
       [<ffffffff81267ab9>] __cpufreq_set_policy+0x184/0x216
       [<ffffffff81268572>] store_scaling_governor+0x1ec/0x228
       [<ffffffff812691fb>] store+0x67/0x8c						
       [<ffffffff81129091>] sysfs_write_file+0xe9/0x11e				
       [<ffffffff810e64d8>] vfs_write+0xb0/0x10a
       [<ffffffff810e6600>] sys_write+0x4c/0x74
       [<ffffffff8100bc1b>] system_call_fastpath+0x16/0x1b
       [<ffffffffffffffff>] 0xffffffffffffffff

other info that might help us debug this:

3 locks held by K99cpuspeed/227488:
 #0:  (&buffer->mutex){+.+.+.}, at: [<ffffffff81128fe5>]
sysfs_write_file+0x3d/0x11e
 #1:  (&per_cpu(cpu_policy_rwsem, cpu)){+++++.}, at:
[<ffffffff81268ad3>] lock_policy_rwsem_write+0x4d/0x7d
 #2:  (dbs_mutex){+.+.+.}, at: [<ffffffffa0081af3>]
cpufreq_governor_dbs+0x241/0x2d2 [cpufreq_ondemand]

stack backtrace:
Pid: 227488, comm: K99cpuspeed Not tainted 2.6.30-rc4-mm1 #26
Call Trace:
 [<ffffffff81069347>] print_circular_bug_tail+0x71/0x7c
 [<ffffffff8106a04e>] __lock_acquire+0x991/0xc33
 [<ffffffff8104e2f6>] ? lock_timer_base+0x2b/0x4f
 [<ffffffff8106a3b1>] lock_acquire+0xc1/0xe5
 [<ffffffff81055bfd>] ? __cancel_work_timer+0xde/0x227
 [<ffffffff81055c36>] __cancel_work_timer+0x117/0x227		
 [<ffffffff81055bfd>] ? __cancel_work_timer+0xde/0x227
 [<ffffffff81068a22>] ? mark_held_locks+0x4d/0x6b
 [<ffffffff81303d5a>] ? __mutex_lock_common+0x302/0x34c
 [<ffffffffa0081af3>] ? cpufreq_governor_dbs+0x241/0x2d2 [cpufreq_ondemand]	
 [<ffffffff81068a22>] ? mark_held_locks+0x4d/0x6b
 [<ffffffffa0081af3>] ? cpufreq_governor_dbs+0x241/0x2d2 [cpufreq_ondemand]
 [<ffffffff8100b956>] ? ftrace_call+0x5/0x2b
 [<ffffffff81055d58>] cancel_delayed_work_sync+0x12/0x14
 [<ffffffffa0081b06>] cpufreq_governor_dbs+0x254/0x2d2 [cpufreq_ondemand]
 [<ffffffff8105ce4d>] ? up_read+0x2b/0x2f
 [<ffffffff812678bd>] __cpufreq_governor+0xa7/0xe4
 [<ffffffff81267ab9>] __cpufreq_set_policy+0x184/0x216
 [<ffffffff81268572>] store_scaling_governor+0x1ec/0x228
 [<ffffffff81269354>] ? handle_update+0x0/0x39
 [<ffffffff812691fb>] store+0x67/0x8c
 [<ffffffff81129091>] sysfs_write_file+0xe9/0x11e
 [<ffffffff810e64d8>] vfs_write+0xb0/0x10a
 [<ffffffff810e6600>] sys_write+0x4c/0x74
 [<ffffffff8100bc1b>] system_call_fastpath+0x16/0x1b
[  OK  ]
Sending all processes th
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