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

Mathieu Desnoyers
2009-06-11T15:23:29Z

* Simon Holm Thøgersen (odie-t5LvXY1cjzpaa/9Udqfwiw@public.gmane.org) 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-scC8bbJcJLCw5LPnMra/2Q@public.gmane.org>
> > > > > 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
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