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SubjectRe: [BUG] While changing the cpufreq governor, kernel hits a bug in workqueue.c
Date
Hi Nageswara,

Johannes Weiner <hannes@saeurebad.de> writes:

> Hi,
>
> Nageswara R Sastry <rnsastry@linux.vnet.ibm.com> writes:
>
>> Hi,
>>
>> Johannes Weiner wrote:
>>
>>> From: Johannes Weiner <hannes@saeurebad.de>
>>> Subject: cpufreq: cancel self-rearming work synchroneuously
>>>
>>> The ondemand and conservative governor workers are self-rearming.
>>> Cancel them synchroneously to avoid nasty races.
>>>
>>> Reported-by: Nageswara R Sastry <rnsastry@linux.vnet.ibm.com>
>>> Signed-off-by: Johannes Weiner <hannes@saeurebad.de>
>>> ---
>>>
>>> diff --git a/drivers/cpufreq/cpufreq_conservative.c b/drivers/cpufreq/cpufreq_conservative.c
>>> index 5d3a04b..78bac06 100644
>>> --- a/drivers/cpufreq/cpufreq_conservative.c
>>> +++ b/drivers/cpufreq/cpufreq_conservative.c
>>> @@ -467,7 +467,7 @@ static inline void dbs_timer_init(void)
>>>
>>> static inline void dbs_timer_exit(void)
>>> {
>>> - cancel_delayed_work(&dbs_work);
>>> + cancel_delayed_work_sync(&dbs_work);
>>> return;
>>> }
>>>
>>> diff --git a/drivers/cpufreq/cpufreq_ondemand.c b/drivers/cpufreq/cpufreq_ondemand.c
>>> index d2af20d..1eb8c58 100644
>>> --- a/drivers/cpufreq/cpufreq_ondemand.c
>>> +++ b/drivers/cpufreq/cpufreq_ondemand.c
>>> @@ -490,7 +490,7 @@ static inline void dbs_timer_init(struct cpu_dbs_info_s *dbs_info)
>>> static inline void dbs_timer_exit(struct cpu_dbs_info_s *dbs_info)
>>> {
>>> dbs_info->enable = 0;
>>> - cancel_delayed_work(&dbs_info->work);
>>> + cancel_delayed_work_sync(&dbs_info->work);
>>> }
>>>
>>> static int cpufreq_governor_dbs(struct cpufreq_policy *policy,
>>
>> Applied the above patch only and compiled the kernel and seeing an
>> Circular lock related issue at the time of booting. First I am
>> checking this and will let you the results by applying both the
>> patches.
>>
>> =======================================================
>> [ INFO: possible circular locking dependency detected ]
>> 2.6.25.7.cpufreq_patch #2
>> -------------------------------------------------------
>> S06cpuspeed/3493 is trying to acquire lock:
>> (&(&dbs_info->work)->work){--..}, at: [<c012f46c>]
>> __cancel_work_timer+0x80/0x177
>>
>> but task is already holding lock:
>> (dbs_mutex){--..}, at: [<c041e7cb>] cpufreq_governor_dbs+0x25e/0x2ed
>>
>> which lock already depends on the new lock.
>>
>>
>> the existing dependency chain (in reverse order) is:
>>
>> -> #2 (dbs_mutex){--..}:
>> [<c013aa76>] add_lock_to_list+0x61/0x83
>> [<c013cfa3>] __lock_acquire+0x953/0xb05
>> [<c041e5e1>] cpufreq_governor_dbs+0x74/0x2ed
>> [<c013d1b4>] lock_acquire+0x5f/0x79
>> [<c041e5e1>] cpufreq_governor_dbs+0x74/0x2ed
>> [<c04cdaa7>] mutex_lock_nested+0xce/0x222
>> [<c041e5e1>] cpufreq_governor_dbs+0x74/0x2ed
>> [<c041e5e1>] cpufreq_governor_dbs+0x74/0x2ed
>> [<c041e5e1>] cpufreq_governor_dbs+0x74/0x2ed
>> [<c041c87a>] __cpufreq_governor+0x73/0xa6
>> [<c041c9e8>] __cpufreq_set_policy+0x13b/0x19e
>> [<c041d6b5>] cpufreq_add_dev+0x3b4/0x4aa
>> [<c041d296>] handle_update+0x0/0x21
>> [<c02ee310>] sysdev_driver_register+0x48/0x9a
>> [<c041c75b>] cpufreq_register_driver+0x9b/0x147
>> [<c06b742c>] kernel_init+0x130/0x26f
>> [<c06b72fc>] kernel_init+0x0/0x26f
>> [<c06b72fc>] kernel_init+0x0/0x26f
>> [<c0105527>] kernel_thread_helper+0x7/0x10
>> [<ffffffff>] 0xffffffff
>>
>> -> #1 (&per_cpu(cpu_policy_rwsem, cpu)){----}:
>> [<c013cfa3>] __lock_acquire+0x953/0xb05
>> [<c041d194>] lock_policy_rwsem_write+0x30/0x56
>> [<c010a83b>] save_stack_trace+0x1a/0x35
>> [<c013d1b4>] lock_acquire+0x5f/0x79
>> [<c041d194>] lock_policy_rwsem_write+0x30/0x56
>> [<c04cdfd9>] down_write+0x2b/0x44
>> [<c041d194>] lock_policy_rwsem_write+0x30/0x56
>> [<c041d194>] lock_policy_rwsem_write+0x30/0x56
>> [<c041e35e>] do_dbs_timer+0x40/0x24f
>> [<c012ee7f>] run_workqueue+0x81/0x187
>> [<c012eeba>] run_workqueue+0xbc/0x187
>> [<c012ee7f>] run_workqueue+0x81/0x187
>> [<c041e31e>] do_dbs_timer+0x0/0x24f
>> [<c012f6fa>] worker_thread+0x0/0xbd
>> [<c012f7ad>] worker_thread+0xb3/0xbd
>> [<c0131acc>] autoremove_wake_function+0x0/0x2d
>> [<c0131a1b>] kthread+0x38/0x5d
>> [<c01319e3>] kthread+0x0/0x5d
>> [<c0105527>] kernel_thread_helper+0x7/0x10
>> [<ffffffff>] 0xffffffff
>>
>> -> #0 (&(&dbs_info->work)->work){--..}:
>> [<c013b6a2>] print_circular_bug_tail+0x2a/0x61
>> [<c013cec8>] __lock_acquire+0x878/0xb05
>> [<c013d1b4>] lock_acquire+0x5f/0x79
>> [<c012f46c>] __cancel_work_timer+0x80/0x177
>> [<c012f497>] __cancel_work_timer+0xab/0x177
>> [<c012f46c>] __cancel_work_timer+0x80/0x177
>> [<c013c0ee>] mark_held_locks+0x39/0x53
>> [<c04cdbe8>] mutex_lock_nested+0x20f/0x222
>> [<c013c277>] trace_hardirqs_on+0xe7/0x10e
>> [<c04cdbf3>] mutex_lock_nested+0x21a/0x222
>> [<c041e7cb>] cpufreq_governor_dbs+0x25e/0x2ed
>> [<c041e7dd>] cpufreq_governor_dbs+0x270/0x2ed
>> [<c041c87a>] __cpufreq_governor+0x73/0xa6
>> [<c041c9d6>] __cpufreq_set_policy+0x129/0x19e
>> [<c041ce0b>] store_scaling_governor+0x112/0x135
>> [<c041d296>] handle_update+0x0/0x21
>> [<c0410065>] atkbd_set_leds+0x9/0xcf
>> [<c041ccf9>] store_scaling_governor+0x0/0x135
>> [<c041d7e7>] store+0x3c/0x54
>> [<c01a09a0>] sysfs_write_file+0xa9/0xdd
>> [<c01a08f7>] sysfs_write_file+0x0/0xdd
>> [<c016e412>] vfs_write+0x83/0xf6
>> [<c016e958>] sys_write+0x3c/0x63
>> [<c0104816>] sysenter_past_esp+0x5f/0xa5
>> [<ffffffff>] 0xffffffff
>>
>> other info that might help us debug this:
>>
>> 3 locks held by S06cpuspeed/3493:
>> #0: (&buffer->mutex){--..}, at: [<c01a091b>] sysfs_write_file+0x24/0xdd
>> #1: (&per_cpu(cpu_policy_rwsem, cpu)){----}, at: [<c041d194>]
>> lock_policy_rwsem_write+0x30/0x56
>> #2: (dbs_mutex){--..}, at: [<c041e7cb>] cpufreq_governor_dbs+0x25e/0x2ed
>>
>> stack backtrace:
>> Pid: 3493, comm: S06cpuspeed Not tainted 2.6.25.7.cpufreq_patch #2
>> [<c013b6cf>] print_circular_bug_tail+0x57/0x61
>> [<c013cec8>] __lock_acquire+0x878/0xb05
>> [<c013d1b4>] lock_acquire+0x5f/0x79
>> [<c012f46c>] __cancel_work_timer+0x80/0x177
>> [<c012f497>] __cancel_work_timer+0xab/0x177
>> [<c012f46c>] __cancel_work_timer+0x80/0x177
>> [<c013c0ee>] mark_held_locks+0x39/0x53
>> [<c04cdbe8>] mutex_lock_nested+0x20f/0x222
>> [<c013c277>] trace_hardirqs_on+0xe7/0x10e
>> [<c04cdbf3>] mutex_lock_nested+0x21a/0x222
>> [<c041e7cb>] cpufreq_governor_dbs+0x25e/0x2ed
>> [<c041e7dd>] cpufreq_governor_dbs+0x270/0x2ed
>> [<c041c87a>] __cpufreq_governor+0x73/0xa6
>> [<c041c9d6>] __cpufreq_set_policy+0x129/0x19e
>> [<c041ce0b>] store_scaling_governor+0x112/0x135
>> [<c041d296>] handle_update+0x0/0x21
>> [<c0410065>] atkbd_set_leds+0x9/0xcf
>> [<c041ccf9>] store_scaling_governor+0x0/0x135
>> [<c041d7e7>] store+0x3c/0x54
>> [<c01a09a0>] sysfs_write_file+0xa9/0xdd
>> [<c01a08f7>] sysfs_write_file+0x0/0xdd
>> [<c016e412>] vfs_write+0x83/0xf6
>> [<c016e958>] sys_write+0x3c/0x63
>> [<c0104816>] sysenter_past_esp+0x5f/0xa5
>> =======================
>
> Okay, the problem is in cpufreq_conservative.c. We
> cancel_delayed_work_sync() while holding the mutex, but the work itself
> tries to grab it and there it deadlocks; lockdep caught that right.
>
> The hunk for _ondemand is correct, but the one for _conservative is
> obviously wrong, sorry :/
>
> I will whip something up and get back to you. Thanks a lot for
> testing!

Could you try the attached patch instead of the one above?

Dave, I dropped the mutex-grabbing from the conservative worker function
as well as I don't see a reason for it, please correct me if I'm wrong.

Hannes

--
From: Johannes Weiner <hannes@saeurebad.de>
Subject: cpufreq: cancel self-rearming work synchroneously

The ondemand and conservative governor workers are self-rearming.
Cancel them synchroneously to avoid nasty races.

This patch also removes taking a mutex in the conservative worker
function as the locking is dbs_mutex -> work and not the
other way round.

Reported-by: Nageswara R Sastry <rnsastry@linux.vnet.ibm.com>
Signed-off-by: Johannes Weiner <hannes@saeurebad.de>
---
drivers/cpufreq/cpufreq_conservative.c | 4 +---
drivers/cpufreq/cpufreq_ondemand.c | 2 +-
2 files changed, 2 insertions(+), 4 deletions(-)

--- a/drivers/cpufreq/cpufreq_conservative.c
+++ b/drivers/cpufreq/cpufreq_conservative.c
@@ -450,12 +450,10 @@ static void dbs_check_cpu(int cpu)
static void do_dbs_timer(struct work_struct *work)
{
int i;
- mutex_lock(&dbs_mutex);
for_each_online_cpu(i)
dbs_check_cpu(i);
schedule_delayed_work(&dbs_work,
usecs_to_jiffies(dbs_tuners_ins.sampling_rate));
- mutex_unlock(&dbs_mutex);
}

static inline void dbs_timer_init(void)
@@ -467,7 +465,7 @@ static inline void dbs_timer_init(void)

static inline void dbs_timer_exit(void)
{
- cancel_delayed_work(&dbs_work);
+ cancel_delayed_work_sync(&dbs_work);
return;
}

--- a/drivers/cpufreq/cpufreq_ondemand.c
+++ b/drivers/cpufreq/cpufreq_ondemand.c
@@ -490,7 +490,7 @@ static inline void dbs_timer_init(struct
static inline void dbs_timer_exit(struct cpu_dbs_info_s *dbs_info)
{
dbs_info->enable = 0;
- cancel_delayed_work(&dbs_info->work);
+ cancel_delayed_work_sync(&dbs_info->work);
}

static int cpufreq_governor_dbs(struct cpufreq_policy *policy,

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