Re: [Bug #13475] suspend/hibernate lockdep warning

[Mathieu Desnoyers]

* Simon Holm Thøgersen (odie@xxxxxxxxx) wrote:
> man, 08 06 2009 kl. 10:32 -0400, skrev Dave Jones: 
> > On Mon, Jun 08, 2009 at 08:48:45AM -0400, Mathieu Desnoyers wrote:
> >  
> >  > > > >> Bug-Entry       : http://bugzilla.kernel.org/show_bug.cgi?id=13475
> >  > > > >> Subject         : suspend/hibernate lockdep warning
> >  > > > >> References      : http://marc.info/?l=linux-kernel&m=124393723321241&w=4
> >  > > > 
> >  > > > I suspect the following commit, after revert this patch I test 5 times
> >  > > > without lockdep warnings.
> >  > > > 
> >  > > > commit b14893a62c73af0eca414cfed505b8c09efc613c
> >  > > > Author: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx>
> >  > > > Date:   Sun May 17 10:30:45 2009 -0400
> >  > > > 
> >  > > > 	[CPUFREQ] fix timer teardown in ondemand governor
> >  > > 
> >  > > The patch is probably not at fault here. I suspect it's some latent bug
> >  > > that simply got exposed by the change to cancel_delayed_work_sync(). In
> >  > > any case, Mathieu, can you take a look at this please?
> >  > 
> >  > Yes, it's been looked at and discussed on the cpufreq ML. The short
> >  > answer is that they plan to re-engineer cpufreq and remove the policy
> >  > rwlock taken around almost every operations at the cpufreq level.
> >  > 
> >  > The short-term solution, which is recognised as ugly, would be do to the
> >  > following before doing the cancel_delayed_work_sync() :
> >  > 
> >  > unlock policy rwlock write lock
> >  > 
> >  > lock policy rwlock write lock
> >  > 
> >  > It basically works because this rwlock is unneeded for teardown, hence
> >  > the future re-work planned.
> >  > 
> >  > I'm sorry I cannot prepare a patch current... I've got quite a few pages
> >  > of Ph.D. thesis due for the beginning of July.
> >  
> > I'm kinda scared to touch this code at all for .30 due to the number of
> > unexpected gotchas we seem to run into every time we touch something
> > locking related.  So I'm inclined to just live with the lockdep warning
> > for .30, and see how the real fixes look for .31, and push them back
> > as -stable updates if they work out.
> 
> Unfortunately I don't think it is just theoretical, I've actually hit
> the following (that haven't got anything to do with suspend/hibernate)
> 
> INFO: task cpufreqd:4676 blocked for more than 120 seconds.
>  "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>  cpufreqd      D eee2ac60     0  4676      1
>   ee01bd68 00000086 eee2aad0 eee2ac60 00000533 eee2aad0 eee2ac60 0002b16f
>   00000000 eee2ac60 7fffffff 7fffffff eee2ac60 7fffffff 7fffffff 00000000
>   ee01bd70 c03117ee ee01bdbc c0311c0c eee2aad0 eecf6900 eee2aad0 eecf6900
>  Call Trace:
>   [<c03117ee>] schedule+0x12/0x24
>   [<c0311c0c>] schedule_timeout+0x17/0x170
>   [<c011a4f7>] ? __wake_up+0x2b/0x51
>   [<c0311afd>] wait_for_common+0xc4/0x135
>   [<c011a694>] ? default_wake_function+0x0/0xd
>   [<c0311be0>] wait_for_completion+0x12/0x14
>   [<c012bc6a>] __cancel_work_timer+0xfe/0x129
>   [<c012b635>] ? wq_barrier_func+0x0/0xd
>   [<c012bca0>] cancel_delayed_work_sync+0xb/0xd
>   [<f20948f9>] cpufreq_governor_dbs+0x22e/0x291 [cpufreq_ondemand]
>   [<c02af857>] __cpufreq_governor+0x65/0x9d
>   [<c02af960>] __cpufreq_set_policy+0xd1/0x11f
>   [<c02b02ae>] store_scaling_governor+0x18a/0x1b2
>   [<c02b09a5>] ? handle_update+0x0/0xd
>   [<c02b0124>] ? store_scaling_governor+0x0/0x1b2
>   [<c02b08c9>] store+0x48/0x61
>   [<c01acbf4>] sysfs_write_file+0xb4/0xdf
>   [<c01acb40>] ? sysfs_write_file+0x0/0xdf
>   [<c0175535>] vfs_write+0x8a/0x104
>   [<c0175648>] sys_write+0x3b/0x60
>   [<c0103110>] sysenter_do_call+0x12/0x2c
>  INFO: task kondemand/0:4956 blocked for more than 120 seconds.
>  "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>  kondemand/0   D 00000533     0  4956      2
>   ee1d9efc 00000046 c011815f 00000533 071148de ee1e0080 ee1e0210 00000000
>   c03ff478 9189e633 00000082 c03ff478 ee1e0210 c04159f4 c04159f0 00000000
>   ee1d9f04 c03117ee ee1d9f28 c0313104 ee1d9f30 c04159f4 ee1e0080 c01183be
>  Call Trace:
>   [<c011815f>] ? update_curr+0x6c/0x14b
>   [<c03117ee>] schedule+0x12/0x24
>   [<c0313104>] rwsem_down_failed_common+0x150/0x16e
>   [<c01183be>] ? dequeue_task_fair+0x51/0x56
>   [<c031313d>] rwsem_down_write_failed+0x1b/0x23
>   [<c031317e>] call_rwsem_down_write_failed+0x6/0x8
>   [<c03125dd>] ? down_write+0x14/0x16
>   [<c02b0460>] lock_policy_rwsem_write+0x1d/0x33
>   [<f20944aa>] do_dbs_timer+0x45/0x266 [cpufreq_ondemand]
>   [<c012b8f7>] worker_thread+0x165/0x212
>   [<f2094465>] ? do_dbs_timer+0x0/0x266 [cpufreq_ondemand]
>   [<c012e639>] ? autoremove_wake_function+0x0/0x33
>   [<c012b792>] ? worker_thread+0x0/0x212
>   [<c012e278>] kthread+0x42/0x67
>   [<c012e236>] ? kthread+0x0/0x67
>   [<c01038eb>] kernel_thread_helper+0x7/0x10
> 
> I've only seen it once in 5 boots and CONFIG_PROVELOCKING does not give any
> warnings about this, though it does yell when switching governor as reported
> by others in bug #13493.
> 
> Let's hope Mathieu nails it, though I know he's busy with his thesis.
> 

Thanks for the lockdep reports,

I'm currently looking into it, and it's not pretty. Basically we have :

A
  B
(means B nested in A)

work
  read rwlock policy

dbs_mutex
  work
    read rwlock policy

write rwlock policy
  dbs_mutex

So the added dbs_mutex <- work <- rwlock policy dependency (for proper
teardown) is firing the reverse dependency between policy rwlock and
dbs_mutex.

The real way to fix this is to do not take the rwlock policy around
non-policy-related actions, like governor START/STOP doing worker
creation/teardown.

One simple short-term solution would be to take a mutex outside of the
policy rwlock write lock in cpufreq.c. This mutex would be the
equivalent of dbs_mutex "lifted" outside of the rwlock write lock. For
teardown, we only need to hold this mutex, not the rwlock write lock.
Then we can remove the dbs_mutex from the governors.

But looking at cpufreq.c's cpufreq_add_dev() is very much like kicking a
wasp nest: a lot of error paths are not handled properly, and I fear
someone will have to go through the code, fix the currently incorrect
code paths, and then add the lifted mutex.

I currently have no time for implementation due to my thesis, but I'll
be happy to review a patch.

Mathieu

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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