[...]

quoted

quoted

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 <redacted>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>

Acked-by: Ulf Hansson <redacted>

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