[PATCH V2] PM / Domains: Fix potential deadlock while adding/removing subdomains

[PATCH V2] PM / Domains: Fix potential deadlock while adding/removing subdomains

[Ulf Hansson]

We must preserve the same order of how we acquire and release the lock for
genpd, as otherwise we may encounter deadlocks.

The power on phase of a genpd starts by acquiring its lock. Then it walks
the hierarchy of its parent domains to be able to power on these first, as
per design of genpd.

>From a locking perspective this means the locks of the parents becomes
acquired after the lock of the subdomain.

Let's fix pm_genpd_add|remove_subdomain() to maintain the same order of
acquiring/releasing the genpd lock as being applied in the power on/off
sequence.

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

Changes in v2:
	Fix lockdep warning.

---
 drivers/base/power/domain.c | 14 ++++++--------
 1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/drivers/base/power/domain.c b/drivers/base/power/domain.c
index 6ac9a7f..676d762 100644
--- a/drivers/base/power/domain.c
+++ b/drivers/base/power/domain.c
@@ -1339,8 +1339,8 @@ int pm_genpd_add_subdomain(struct generic_pm_domain *genpd,
 	if (!link)
 		return -ENOMEM;
 
-	mutex_lock(&genpd->lock);
-	mutex_lock_nested(&subdomain->lock, SINGLE_DEPTH_NESTING);
+	mutex_lock(&subdomain->lock);
+	mutex_lock_nested(&genpd->lock, SINGLE_DEPTH_NESTING);
 
 	if (genpd->status == GPD_STATE_POWER_OFF
 	    &&  subdomain->status != GPD_STATE_POWER_OFF) {
@@ -1363,8 +1363,8 @@ int pm_genpd_add_subdomain(struct generic_pm_domain *genpd,
 		genpd_sd_counter_inc(genpd);
 
  out:
-	mutex_unlock(&subdomain->lock);
 	mutex_unlock(&genpd->lock);
+	mutex_unlock(&subdomain->lock);
 	if (ret)
 		kfree(link);
 	return ret;
@@ -1385,7 +1385,8 @@ int pm_genpd_remove_subdomain(struct generic_pm_domain *genpd,
 	if (IS_ERR_OR_NULL(genpd) || IS_ERR_OR_NULL(subdomain))
 		return -EINVAL;
 
-	mutex_lock(&genpd->lock);
+	mutex_lock(&subdomain->lock);
+	mutex_lock_nested(&genpd->lock, SINGLE_DEPTH_NESTING);
 
 	if (!list_empty(&subdomain->slave_links) || subdomain->device_count) {
 		pr_warn("%s: unable to remove subdomain %s\n", genpd->name,
@@ -1398,22 +1399,19 @@ int pm_genpd_remove_subdomain(struct generic_pm_domain *genpd,
 		if (link->slave != subdomain)
 			continue;
 
-		mutex_lock_nested(&subdomain->lock, SINGLE_DEPTH_NESTING);
-
 		list_del(&link->master_node);
 		list_del(&link->slave_node);
 		kfree(link);
 		if (subdomain->status != GPD_STATE_POWER_OFF)
 			genpd_sd_counter_dec(genpd);
 
-		mutex_unlock(&subdomain->lock);
-
 		ret = 0;
 		break;
 	}
 
 out:
 	mutex_unlock(&genpd->lock);
+	mutex_unlock(&subdomain->lock);
 
 	return ret;
 }
-- 
1.9.1

Re: [PATCH V2] PM / Domains: Fix potential deadlock while adding/removing subdomains

[Rafael J. Wysocki]

On Wednesday, January 20, 2016 12:07:39 PM Ulf Hansson wrote:
> We must preserve the same order of how we acquire and release the lock for
> genpd, as otherwise we may encounter deadlocks.
> 
> The power on phase of a genpd starts by acquiring its lock. Then it walks
> the hierarchy of its parent domains to be able to power on these first, as
> per design of genpd.
> 
> From a locking perspective this means the locks of the parents becomes
> acquired after the lock of the subdomain.
> 
> Let's fix pm_genpd_add|remove_subdomain() to maintain the same order of
> acquiring/releasing the genpd lock as being applied in the power on/off
> sequence.
> 
> Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

Applied, thanks!

Rafael

Re: [PATCH V2] PM / Domains: Fix potential deadlock while adding/removing subdomains

[Geert Uytterhoeven]

Hi Rafael,

On Sun, Jan 24, 2016 at 2:20 AM, Rafael J. Wysocki <rjw@rjwysocki.net> wrote:
> On Wednesday, January 20, 2016 12:07:39 PM Ulf Hansson wrote:
>> We must preserve the same order of how we acquire and release the lock for
>> genpd, as otherwise we may encounter deadlocks.
>>
>> The power on phase of a genpd starts by acquiring its lock. Then it walks
>> the hierarchy of its parent domains to be able to power on these first, as
>> per design of genpd.
>>
>> From a locking perspective this means the locks of the parents becomes
>> acquired after the lock of the subdomain.
>>
>> Let's fix pm_genpd_add|remove_subdomain() to maintain the same order of
>> acquiring/releasing the genpd lock as being applied in the power on/off
>> sequence.
>>
>> Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
>
> Applied, thanks!

It looks like you accidentally applied v1, introducing the lockdep warnings
fixed in v2?

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds

Re: [PATCH V2] PM / Domains: Fix potential deadlock while adding/removing subdomains

[Ulf Hansson]

On 26 January 2016 at 14:13, Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> Hi Rafael,
>
> On Sun, Jan 24, 2016 at 2:20 AM, Rafael J. Wysocki <rjw@rjwysocki.net> wrote:
>> On Wednesday, January 20, 2016 12:07:39 PM Ulf Hansson wrote:
>>> We must preserve the same order of how we acquire and release the lock for
>>> genpd, as otherwise we may encounter deadlocks.
>>>
>>> The power on phase of a genpd starts by acquiring its lock. Then it walks
>>> the hierarchy of its parent domains to be able to power on these first, as
>>> per design of genpd.
>>>
>>> From a locking perspective this means the locks of the parents becomes
>>> acquired after the lock of the subdomain.
>>>
>>> Let's fix pm_genpd_add|remove_subdomain() to maintain the same order of
>>> acquiring/releasing the genpd lock as being applied in the power on/off
>>> sequence.
>>>
>>> Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
>>
>> Applied, thanks!
>
> It looks like you accidentally applied v1, introducing the lockdep warnings
> fixed in v2?
>

Correct, it's the v1.

Rafael, if you can't replace the patch I can send an incremental fix
on top. Just tell me what way you prefer.

Kind regards
Uffe

Re: [PATCH V2] PM / Domains: Fix potential deadlock while adding/removing subdomains

[Rafael J. Wysocki]

On Tuesday, January 26, 2016 03:59:19 PM Ulf Hansson wrote:
> On 26 January 2016 at 14:13, Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> > Hi Rafael,
> >
> > On Sun, Jan 24, 2016 at 2:20 AM, Rafael J. Wysocki <rjw@rjwysocki.net> wrote:
> >> On Wednesday, January 20, 2016 12:07:39 PM Ulf Hansson wrote:
> >>> We must preserve the same order of how we acquire and release the lock for
> >>> genpd, as otherwise we may encounter deadlocks.
> >>>
> >>> The power on phase of a genpd starts by acquiring its lock. Then it walks
> >>> the hierarchy of its parent domains to be able to power on these first, as
> >>> per design of genpd.
> >>>
> >>> From a locking perspective this means the locks of the parents becomes
> >>> acquired after the lock of the subdomain.
> >>>
> >>> Let's fix pm_genpd_add|remove_subdomain() to maintain the same order of
> >>> acquiring/releasing the genpd lock as being applied in the power on/off
> >>> sequence.
> >>>
> >>> Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
> >>
> >> Applied, thanks!
> >
> > It looks like you accidentally applied v1, introducing the lockdep warnings
> > fixed in v2?
> >
> 
> Correct, it's the v1.
> 
> Rafael, if you can't replace the patch I can send an incremental fix
> on top. Just tell me what way you prefer.

Can you please send the whole patch again?

Thanks,
Rafael