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Re: [PATCH] PM / Domains: Release mutex when powering on master domain

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On 22 December 2015 at 14:13, Daniel Kurtz <djkurtz@chromium.org> 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@vger.kernel.org
> Fixes: 5d837eef7b99 ("PM / Domains: Remove intermediate states from the power off sequence")
> Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
> ---
>  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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[PATCH] PM / Domains: Release mutex when powering on master domain Daniel Kurtz <djkurtz@chromium.org> - 2015-12-22 14:20 +0100
  Re: [PATCH] PM / Domains: Release mutex when powering on master domain Kevin Hilman <khilman@deeprootsystems.com> - 2015-12-22 15:30 +0100
  Re: [PATCH] PM / Domains: Release mutex when powering on master domain Ulf Hansson <ulf.hansson@linaro.org> - 2015-12-23 13:00 +0100

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