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

[Ulf Hansson <ulf.hansson@linaro.org>]

[...]

>>> 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>
>>

Acked-by: Ulf Hansson <ulf.hansson@linaro.org>

You didn't send this to linux-pm so probably you should resend it, so
Rafael can pick it up.

[...]

>
> 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.

:-) Of course you are absolutely right. Thanks!

[...]

Kind regards
Uffe