On 22 December 2015 at 14:13, Daniel Kurtz <djkurtz@xxxxxxxxxxxx> wrote:
> Commit ba2bbfbf6307 (PM / Domains: Remove intermediate states from the
> power off sequence) removed the mutex_unlock()/_lock() around powering on
> a genpd's master domain in __genpd_poweron().
>
> Since all genpd's share a mutex lockdep class, this causes a "possible
> recursive locking detected" lockdep warning on boot when trying to power
> on a genpd slave domain:
>
> [ 1.893137] =============================================
> [ 1.893139] [ INFO: possible recursive locking detected ]
> [ 1.893143] 3.18.0 #531 Not tainted
> [ 1.893145] ---------------------------------------------
> [ 1.893148] kworker/u8:4/113 is trying to acquire lock:
> [ 1.893167] (&genpd->lock){+.+...}, at: [<ffffffc000573818>] genpd_poweron+0x30/0x70
> [ 1.893169]
> [ 1.893169] but task is already holding lock:
> [ 1.893179] (&genpd->lock){+.+...}, at: [<ffffffc000573818>] genpd_poweron+0x30/0x70
> [ 1.893182]
> [ 1.893182] other info that might help us debug this:
> [ 1.893184] Possible unsafe locking scenario:
> [ 1.893184]
> [ 1.893185] CPU0
> [ 1.893187] ----
> [ 1.893191] lock(&genpd->lock);
> [ 1.893195] lock(&genpd->lock);
> [ 1.893196]
> [ 1.893196] *** DEADLOCK ***
> [ 1.893196]
> [ 1.893198] May be due to missing lock nesting notation
> [ 1.893198]
> [ 1.893201] 4 locks held by kworker/u8:4/113:
> [ 1.893217] #0: ("%s""deferwq"){++++.+}, at: [<ffffffc00023b4e0>] process_one_work+0x1f8/0x50c
> [ 1.893229] #1: (deferred_probe_work){+.+.+.}, at: [<ffffffc00023b4e0>] process_one_work+0x1f8/0x50c
> [ 1.893241] #2: (&dev->mutex){......}, at: [<ffffffc000560920>] __device_attach+0x40/0x12c
> [ 1.893251] #3: (&genpd->lock){+.+...}, at: [<ffffffc000573818>] genpd_poweron+0x30/0x70
> [ 1.893253]
> [ 1.893253] stack backtrace:
> [ 1.893259] CPU: 2 PID: 113 Comm: kworker/u8:4 Not tainted 3.18.0 #531
> [ 1.893269] Workqueue: deferwq deferred_probe_work_func
> [ 1.893271] Call trace:
> [ 1.893295] [<ffffffc000269dcc>] __lock_acquire+0x68c/0x19a8
> [ 1.893299] [<ffffffc00026b954>] lock_acquire+0x128/0x164
> [ 1.893304] [<ffffffc00084e090>] mutex_lock_nested+0x90/0x3b4
> [ 1.893308] [<ffffffc000573814>] genpd_poweron+0x2c/0x70
> [ 1.893312] [<ffffffc0005738ac>] __genpd_poweron.part.14+0x54/0xcc
> [ 1.893316] [<ffffffc000573834>] genpd_poweron+0x4c/0x70
> [ 1.893321] [<ffffffc00057447c>] genpd_dev_pm_attach+0x160/0x19c
> [ 1.893326] [<ffffffc00056931c>] dev_pm_domain_attach+0x1c/0x2c
> ...
>
> Fix this by releasing the slaves mutex before acquiring the master's,
> which restores the old behavior.
>
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: 5d837eef7b99 ("PM / Domains: Remove intermediate states from the power off sequence")
> Signed-off-by: Daniel Kurtz <djkurtz@xxxxxxxxxxxx>
> ---
> drivers/base/power/domain.c | 5 +++++
> 1 file changed, 5 insertions(+)
>
> diff --git a/drivers/base/power/domain.c b/drivers/base/power/domain.c
> index 65f50ec..56fa335 100644
> --- a/drivers/base/power/domain.c
> +++ b/drivers/base/power/domain.c
> @@ -196,7 +196,12 @@ static int __genpd_poweron(struct generic_pm_domain *genpd)
> list_for_each_entry(link, &genpd->slave_links, slave_node) {
> genpd_sd_counter_inc(link->master);
>
> + mutex_unlock(&genpd->lock);
> +
> ret = genpd_poweron(link->master);
> +
> + mutex_lock(&genpd->lock);
> +
> if (ret) {
> genpd_sd_counter_dec(link->master);
> goto err;
> --
> 2.6.0.rc2.230.g3dd15c0
>
As we no longer have protection to deal with intermediate power
states, releasing the lock would mean that __genpd_poweron() can be
called for the same genpd as we just were operating on.
Since the genpd->status hasn't become GPD_STATE_ACTIVE yet, that means
a new power up cycle may start. For example causing the atomic
subdomain count to increase once more. Not good. :-)
So, this approach doesn't work.
Kind regards
Uffe
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