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

From: Thomas Renninger
Date: Wed Jun 17 2009 - 11:29:37 EST

On Wednesday 17 June 2009 02:39:25 Pallipadi, Venkatesh wrote:
> On Thu, Jun 11, 2009 at 08:23:29AM -0700, Mathieu Desnoyers wrote:
> > * 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.
> >
>
> 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
Why is sysfs tunables protection needed at all?

The ondemand locking very much looks like taken over from the userspace
governor. There you need the lock because a write to set_speed directly
calls ->target.

What is urgently missing is a description for what the locks are
really used, not only in which case they deadlock.

From your comment above:
> dbs_mutex does not have anything to do with timer_init and timer_exit.
But this is what it seems to do?
If it's not needed to protect calling timer_init while in timer_exit
(or the other way around) and sysfs_create_group while
in sysfs_remove_group I think the mutex can be deleted.
What do you think about this patch (compile tested only and not
for .30)?

Is someone aware of any test scenarios I could run to try without
the mutex and run into trouble?
Do I totally miss something here or does this make sense?

Thanks,

Thomas

-----

CPUFREQ ondemand: Remove unneeded dbs_mutex

There is no need to protect general (not per core) ondemand sysfs variables
against per core governor (de-)activation (GOV_START/GOV_STOP).

It must just be assured that these are only initialized once, before userspace
can modify them (otherwise userspace modifications will be overriden by
re-initializing the general variables).
This should already be the case.

Signed-off-by: Thomas Renninger <trenn@xxxxxxx>

---
drivers/cpufreq/cpufreq_ondemand.c | 64 +++++++------------------------------
1 file changed, 13 insertions(+), 51 deletions(-)

Index: linux-2.6.29-master/drivers/cpufreq/cpufreq_ondemand.c
===================================================================
--- linux-2.6.29-master.orig/drivers/cpufreq/cpufreq_ondemand.c
+++ linux-2.6.29-master/drivers/cpufreq/cpufreq_ondemand.c
@@ -17,7 +17,6 @@
#include <linux/cpu.h>
#include <linux/jiffies.h>
#include <linux/kernel_stat.h>
-#include <linux/mutex.h>
#include <linux/hrtimer.h>
#include <linux/tick.h>
#include <linux/ktime.h>
@@ -91,16 +90,6 @@ static DEFINE_PER_CPU(struct cpu_dbs_inf

static unsigned int dbs_enable; /* number of CPUs using this policy */

-/*
- * DEADLOCK ALERT! There is a ordering requirement between cpu_hotplug
- * lock and dbs_mutex. cpu_hotplug lock should always be held before
- * dbs_mutex. If any function that can potentially take cpu_hotplug lock
- * (like __cpufreq_driver_target()) is being called with dbs_mutex taken, then
- * cpu_hotplug lock should be taken before that. Note that cpu_hotplug lock
- * is recursive for the same process. -Venki
- */
-static DEFINE_MUTEX(dbs_mutex);
-
static struct workqueue_struct *kondemand_wq;

static struct dbs_tuners {
@@ -266,14 +255,7 @@ static ssize_t store_sampling_rate(struc
int ret;
ret = sscanf(buf, "%u", &input);

- mutex_lock(&dbs_mutex);
- if (ret != 1) {
- mutex_unlock(&dbs_mutex);
- return -EINVAL;
- }
dbs_tuners_ins.sampling_rate = max(input, minimum_sampling_rate());
- mutex_unlock(&dbs_mutex);
-
return count;
}

@@ -284,16 +266,12 @@ static ssize_t store_up_threshold(struct
int ret;
ret = sscanf(buf, "%u", &input);

- mutex_lock(&dbs_mutex);
if (ret != 1 || input > MAX_FREQUENCY_UP_THRESHOLD ||
input < MIN_FREQUENCY_UP_THRESHOLD) {
- mutex_unlock(&dbs_mutex);
return -EINVAL;
}

dbs_tuners_ins.up_threshold = input;
- mutex_unlock(&dbs_mutex);
-
return count;
}

@@ -312,9 +290,7 @@ static ssize_t store_ignore_nice_load(st
if (input > 1)
input = 1;

- mutex_lock(&dbs_mutex);
if (input == dbs_tuners_ins.ignore_nice) { /* nothing to do */
- mutex_unlock(&dbs_mutex);
return count;
}
dbs_tuners_ins.ignore_nice = input;
@@ -329,8 +305,6 @@ static ssize_t store_ignore_nice_load(st
dbs_info->prev_cpu_nice = kstat_cpu(j).cpustat.nice;

}
- mutex_unlock(&dbs_mutex);
-
return count;
}

@@ -347,11 +321,8 @@ static ssize_t store_powersave_bias(stru
if (input > 1000)
input = 1000;

- mutex_lock(&dbs_mutex);
dbs_tuners_ins.powersave_bias = input;
ondemand_powersave_bias_init();
- mutex_unlock(&dbs_mutex);
-
return count;
}

@@ -580,16 +551,6 @@ static int cpufreq_governor_dbs(struct c
if (this_dbs_info->enable) /* Already enabled */
break;

- mutex_lock(&dbs_mutex);
- dbs_enable++;
-
- rc = sysfs_create_group(&policy->kobj, &dbs_attr_group);
- if (rc) {
- dbs_enable--;
- mutex_unlock(&dbs_mutex);
- return rc;
- }
-
for_each_cpu(j, policy->cpus) {
struct cpu_dbs_info_s *j_dbs_info;
j_dbs_info = &per_cpu(cpu_dbs_info, j);
@@ -604,10 +565,10 @@ static int cpufreq_governor_dbs(struct c
}
this_dbs_info->cpu = cpu;
/*
- * Start the timerschedule work, when this governor
- * is used for first time
+ * Initialize general ondemand tunables only ones, not for
+ * each core
*/
- if (dbs_enable == 1) {
+ if (!dbs_enable) {
unsigned int latency;
/* policy latency is in nS. Convert it to uS first */
latency = policy->cpuinfo.transition_latency / 1000;
@@ -619,30 +580,31 @@ static int cpufreq_governor_dbs(struct c
MIN_STAT_SAMPLING_RATE);

dbs_tuners_ins.sampling_rate = def_sampling_rate;
+ }
+ rc = sysfs_create_group(&policy->kobj, &dbs_attr_group);
+ if (rc) {
+ this_dbs_info->enable = 0;
+ return rc;
}
dbs_timer_init(this_dbs_info);
-
- mutex_unlock(&dbs_mutex);
+ dbs_enable++;
break;

case CPUFREQ_GOV_STOP:
- mutex_lock(&dbs_mutex);
- dbs_timer_exit(this_dbs_info);
- sysfs_remove_group(&policy->kobj, &dbs_attr_group);
+ if (this_dbs_info->enable) {
+ dbs_timer_exit(this_dbs_info);
+ sysfs_remove_group(&policy->kobj, &dbs_attr_group);
+ }
dbs_enable--;
- mutex_unlock(&dbs_mutex);
-
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;

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