Date:	Tue, 16 Jun 2009 21:05:40 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>
Cc:	Simon Holm Thøgersen <odie@...aau.dk>,
	Dave Jones <davej@...hat.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Dave Young <hidave.darkstar@...il.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Kernel Testers List <kernel-testers@...r.kernel.org>,
	"cpufreq@...r.kernel.org" <cpufreq@...r.kernel.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	"trenn@...e.de" <trenn@...e.de>,
	"sven.wegener@...aler.net" <sven.wegener@...aler.net>
Subject: Re: [Bug #13475] suspend/hibernate lockdep warning

* Pallipadi, Venkatesh (venkatesh.pallipadi@...el.com) wrote:
> On Thu, Jun 11, 2009 at 08:23:29AM -0700, Mathieu Desnoyers wrote:
> > * Simon Holm Thøgersen (odie@...aau.dk) 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@...ymtl.ca>
> > > >  > > > 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.
> > 
> 
> How about below patch on top of Mathieu's patch here
> http://marc.info/?l=linux-kernel&m=124448150529838&w=2
> 
> [PATCH] cpufreq: Eliminate lockdep issue with dbs_mutex and policy_rwsem
> 
> This removes the unneeded dependency of 
> write rwlock policy
>   dbs_mutex
> 
> dbs_mutex does not have anything to do with timer_init and timer_exit. It
> is just to protect dbs tunables in sysfs cpufreq/ondemand and is not
> needed to be held during timer init, exit as well as during governor limit
> changes.
> 

If this works, then it will likely need to be ported to the conservative
governor too.

Thanks,

Mathieu

> Signed-off-by: Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>
> ---
>  drivers/cpufreq/cpufreq_ondemand.c |    8 +++-----
>  1 files changed, 3 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/cpufreq/cpufreq_ondemand.c b/drivers/cpufreq/cpufreq_ondemand.c
> index e741c33..1c94ff5 100644
> --- a/drivers/cpufreq/cpufreq_ondemand.c
> +++ b/drivers/cpufreq/cpufreq_ondemand.c
> @@ -352,8 +352,8 @@ static ssize_t store_powersave_bias(struct cpufreq_policy *unused,
>  
>  	mutex_lock(&dbs_mutex);
>  	dbs_tuners_ins.powersave_bias = input;
> -	ondemand_powersave_bias_init();
>  	mutex_unlock(&dbs_mutex);
> +	ondemand_powersave_bias_init();
>  
>  	return count;
>  }
> @@ -626,14 +626,14 @@ static int cpufreq_governor_dbs(struct cpufreq_policy *policy,
>  
>  			dbs_tuners_ins.sampling_rate = def_sampling_rate;
>  		}
> +		mutex_unlock(&dbs_mutex);
>  		dbs_timer_init(this_dbs_info);
>  
> -		mutex_unlock(&dbs_mutex);
>  		break;
>  
>  	case CPUFREQ_GOV_STOP:
> -		mutex_lock(&dbs_mutex);
>  		dbs_timer_exit(this_dbs_info);
> +		mutex_lock(&dbs_mutex);
>  		sysfs_remove_group(&policy->kobj, &dbs_attr_group);
>  		dbs_enable--;
>  		mutex_unlock(&dbs_mutex);
> @@ -641,14 +641,12 @@ static int cpufreq_governor_dbs(struct cpufreq_policy *policy,
>  		break;
>  
>  	case CPUFREQ_GOV_LIMITS:
> -		mutex_lock(&dbs_mutex);
>  		if (policy->max < this_dbs_info->cur_policy->cur)
>  			__cpufreq_driver_target(this_dbs_info->cur_policy,
>  				policy->max, CPUFREQ_RELATION_H);
>  		else if (policy->min > this_dbs_info->cur_policy->cur)
>  			__cpufreq_driver_target(this_dbs_info->cur_policy,
>  				policy->min, CPUFREQ_RELATION_L);
> -		mutex_unlock(&dbs_mutex);
>  		break;
>  	}
>  	return 0;
> -- 
> 1.6.0.6
> 

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