Re: [PATCH] power: genpd: fix lockdep issue for all subdomains

[Marek Szyprowski <m.szyprowski@samsung.com>]

Hello,

On 2016-01-19 16:25, Ulf Hansson wrote:
> +linux-pm, Daniel Kurtz, Nicolas Boichat
>
> On 4 January 2016 at 11:39, Marek Szyprowski <m.szyprowski@samsung.com> wrote:
>> During genpd_poweron, genpd->lock is acquired recursively for each
>> parent (master) domain, which are separate obejcts. This confuses
>> lockdep, which considers every operation on genpd->lock as being done on
>> the same lock class. This leads to the following false positive warning:
>>
>> =============================================
>> [ INFO: possible recursive locking detected ]
>> 4.4.0-rc4-xu3s #32 Not tainted
>> ---------------------------------------------
>> swapper/0/1 is trying to acquire lock:
>>   (&genpd->lock){+.+...}, at: [<c0361550>] __genpd_poweron+0x64/0x108
>>
>> but task is already holding lock:
>>   (&genpd->lock){+.+...}, at: [<c0361af8>] genpd_dev_pm_attach+0x168/0x1b8
>>
>> other info that might help us debug this:
>>   Possible unsafe locking scenario:
>>
>>         CPU0
>>         ----
>>    lock(&genpd->lock);
>>    lock(&genpd->lock);
>>
>>   *** DEADLOCK ***
>>
>>   May be due to missing lock nesting notation
>>
>> 3 locks held by swapper/0/1:
>>   #0:  (&dev->mutex){......}, at: [<c0350910>] __driver_attach+0x48/0x98
>>   #1:  (&dev->mutex){......}, at: [<c0350920>] __driver_attach+0x58/0x98
>>   #2:  (&genpd->lock){+.+...}, at: [<c0361af8>] genpd_dev_pm_attach+0x168/0x1b8
>>
>> stack backtrace:
>> CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.4.0-rc4-xu3s #32
>> Hardware name: SAMSUNG EXYNOS (Flattened Device Tree)
>> [<c0016c98>] (unwind_backtrace) from [<c00139c4>] (show_stack+0x10/0x14)
>> [<c00139c4>] (show_stack) from [<c0270df0>] (dump_stack+0x84/0xc4)
>> [<c0270df0>] (dump_stack) from [<c00780b8>] (__lock_acquire+0x1f88/0x215c)
>> [<c00780b8>] (__lock_acquire) from [<c007886c>] (lock_acquire+0xa4/0xd0)
>> [<c007886c>] (lock_acquire) from [<c0641f2c>] (mutex_lock_nested+0x70/0x4d4)
>> [<c0641f2c>] (mutex_lock_nested) from [<c0361550>] (__genpd_poweron+0x64/0x108)
>> [<c0361550>] (__genpd_poweron) from [<c0361b00>] (genpd_dev_pm_attach+0x170/0x1b8)
>> [<c0361b00>] (genpd_dev_pm_attach) from [<c03520a8>] (platform_drv_probe+0x2c/0xac)
>> [<c03520a8>] (platform_drv_probe) from [<c03507d4>] (driver_probe_device+0x208/0x2fc)
>> [<c03507d4>] (driver_probe_device) from [<c035095c>] (__driver_attach+0x94/0x98)
>> [<c035095c>] (__driver_attach) from [<c034ec14>] (bus_for_each_dev+0x68/0x9c)
>> [<c034ec14>] (bus_for_each_dev) from [<c034fec8>] (bus_add_driver+0x1a0/0x218)
>> [<c034fec8>] (bus_add_driver) from [<c035115c>] (driver_register+0x78/0xf8)
>> [<c035115c>] (driver_register) from [<c0338488>] (exynos_drm_register_drivers+0x28/0x74)
>> [<c0338488>] (exynos_drm_register_drivers) from [<c0338594>] (exynos_drm_init+0x6c/0xc4)
>> [<c0338594>] (exynos_drm_init) from [<c00097f4>] (do_one_initcall+0x90/0x1dc)
>> [<c00097f4>] (do_one_initcall) from [<c0895e08>] (kernel_init_freeable+0x158/0x1f8)
>> [<c0895e08>] (kernel_init_freeable) from [<c063ecac>] (kernel_init+0x8/0xe8)
>> [<c063ecac>] (kernel_init) from [<c000f7d0>] (ret_from_fork+0x14/0x24)
>>
>> This patch replaces mutex_lock with mutex_lock_nested() and uses
>> recursion depth to annotate each genpd->lock operation with separate
>> lockdep subclass.
>>
>> Reported-by: Anand Moon <linux.amoon@gmail.com>
>> Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
> Thanks for looking into this!
>
> Daniel Kurtz, did also run into this issue [1] and reported it a while ago.
> There where some discussions and Daniel also posted a patch trying to
> solve the issue [2]. That approach didn't work out, and unfortunate I
> haven't yet been able to look closer into the issue. Sorry about that!
>
>> ---
>>   drivers/base/power/domain.c | 21 +++++++++++++--------
>>   1 file changed, 13 insertions(+), 8 deletions(-)
>>
>> diff --git a/drivers/base/power/domain.c b/drivers/base/power/domain.c
>> index b803790..e02ddf6 100644
>> --- a/drivers/base/power/domain.c
>> +++ b/drivers/base/power/domain.c
>> @@ -170,16 +170,15 @@ static void genpd_queue_power_off_work(struct generic_pm_domain *genpd)
>>          queue_work(pm_wq, &genpd->power_off_work);
>>   }
>>
>> -static int genpd_poweron(struct generic_pm_domain *genpd);
>> -
>>   /**
>>    * __genpd_poweron - Restore power to a given PM domain and its masters.
>>    * @genpd: PM domain to power up.
>> + * @depth: nesting count for lockdep.
>>    *
>>    * Restore power to @genpd and all of its masters so that it is possible to
>>    * resume a device belonging to it.
>>    */
>> -static int __genpd_poweron(struct generic_pm_domain *genpd)
>> +static int __genpd_poweron(struct generic_pm_domain *genpd, unsigned int depth)
>>   {
>>          struct gpd_link *link;
>>          int ret = 0;
>> @@ -194,11 +193,16 @@ static int __genpd_poweron(struct generic_pm_domain *genpd)
>>           * with it.
>>           */
>>          list_for_each_entry(link, &genpd->slave_links, slave_node) {
>> -               genpd_sd_counter_inc(link->master);
>> +               struct generic_pm_domain *master = link->master;
>> +
>> +               genpd_sd_counter_inc(master);
>> +
>> +               mutex_lock_nested(&master->lock, depth + 1);
> Nested locks isn't a solution to a problem, but instead this tricks
> lockdep about what goes on. Right?

The only difference between mutex_lock_nested and mutex_lock is the way
it is interpreted by deplock. The additional argument is deplock subclass
of the lock. The name of this function is imho a bit misleading.

> I am wondering whether there's another option available which better
> can instruct lockdep to not treat this as an error.

Similar solution is already applied in regulators and i2c cores, see
regulator_lock_supply() and i2c_del_adapter() functions.

>> +               ret = __genpd_poweron(master, depth + 1);
>> +               mutex_unlock(&master->lock);
>>
>> -               ret = genpd_poweron(link->master);
>>                  if (ret) {
>> -                       genpd_sd_counter_dec(link->master);
>> +                       genpd_sd_counter_dec(master);
>>                          goto err;
>>                  }
>>          }
>> @@ -230,11 +234,12 @@ static int genpd_poweron(struct generic_pm_domain *genpd)
>>          int ret;
>>
>>          mutex_lock(&genpd->lock);
>> -       ret = __genpd_poweron(genpd);
>> +       ret = __genpd_poweron(genpd, 0);
>>          mutex_unlock(&genpd->lock);
>>          return ret;
>>   }
>>
>> +
>>   static int genpd_save_dev(struct generic_pm_domain *genpd, struct device *dev)
>>   {
>>          return GENPD_DEV_CALLBACK(genpd, int, save_state, dev);
>> @@ -482,7 +487,7 @@ static int pm_genpd_runtime_resume(struct device *dev)
>>          }
>>
>>          mutex_lock(&genpd->lock);
>> -       ret = __genpd_poweron(genpd);
>> +       ret = __genpd_poweron(genpd, 0);
>>          mutex_unlock(&genpd->lock);
>>
>>          if (ret)
>> --
>> 1.9.2
>>
> Kind regards
> Uffe
>
> [1]
> http://www.spinics.net/lists/arm-kernel/msg471025.html
> [2]
> http://www.spinics.net/lists/stable/msg113650.html
>
>

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland