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[PATCH] PM / OPP: Call notifier without holding opp_table->lock

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  [PATCH] PM / OPP: Call notifier without holding opp_table->lock Viresh Kumar <viresh.kumar@linaro.org> - 2017-09-20 17:40 +0200
    Re: [PATCH] PM / OPP: Call notifier without holding opp_table->lock Viresh Kumar <viresh.kumar@linaro.org> - 2017-09-20 19:10 +0200
      Re: [PATCH] PM / OPP: Call notifier without holding opp_table->lock Stephen Boyd <sboyd@codeaurora.org> - 2017-09-20 21:50 +0200
    Re: [PATCH] PM / OPP: Call notifier without holding opp_table->lock Stephen Boyd <sboyd@codeaurora.org> - 2017-09-20 19:10 +0200

#1735853 — [PATCH] PM / OPP: Call notifier without holding opp_table->lock

The notifier callbacks may want to call some OPP helper routines which
may try to take the same opp_table->lock again and cause a deadlock. One
such usecase was reported by Chanwoo Choi, where calling
dev_pm_opp_disable() leads us to the devfreq's OPP notifier handler,
which further calls dev_pm_opp_find_freq_floor() and it deadlocks.

We don't really need the opp_table->lock to be held across the notifier
call though, all we want to make sure is that the 'opp' doesn't get
freed while being used from within the notifier chain. We can do it with
help of dev_pm_opp_get/put() as well. Lets do it.

Reported-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
---
 drivers/base/power/opp/core.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/base/power/opp/core.c b/drivers/base/power/opp/core.c
index 4360b4efcd4c..668fd940d362 100644
--- a/drivers/base/power/opp/core.c
+++ b/drivers/base/power/opp/core.c
@@ -1627,6 +1627,9 @@ static int _opp_set_availability(struct device *dev, unsigned long freq,
 
 	opp->available = availability_req;
 
+	dev_pm_opp_get(opp);
+	mutex_unlock(&opp_table->lock);
+
 	/* Notify the change of the OPP availability */
 	if (availability_req)
 		blocking_notifier_call_chain(&opp_table->head, OPP_EVENT_ENABLE,
@@ -1635,8 +1638,12 @@ static int _opp_set_availability(struct device *dev, unsigned long freq,
 		blocking_notifier_call_chain(&opp_table->head,
 					     OPP_EVENT_DISABLE, opp);
 
+	dev_pm_opp_put(opp);
+	goto put_table;
+
 unlock:
 	mutex_unlock(&opp_table->lock);
+put_table:
 	dev_pm_opp_put_opp_table(opp_table);
 	return r;
 }
-- 
2.7.4

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#1735925

On 20-09-17, 10:00, Stephen Boyd wrote:
> On 09/20, Viresh Kumar wrote:

> > diff --git a/drivers/base/power/opp/core.c b/drivers/base/power/opp/core.c
> > index 4360b4efcd4c..668fd940d362 100644
> > --- a/drivers/base/power/opp/core.c
> > +++ b/drivers/base/power/opp/core.c
> > @@ -1627,6 +1627,9 @@ static int _opp_set_availability(struct device *dev, unsigned long freq,
> >  
> >  	opp->available = availability_req;
> >  
> > +	dev_pm_opp_get(opp);
> > +	mutex_unlock(&opp_table->lock);
> 
> Does this prevent the OPP from changing while the lock is
> released?

No, its just ref counting and will only prevent it from getting freed.

There is only one thing that can change for an OPP though after it is
created, its availability.

> That would be the only difference from before. It's
> possible that nobody cares about this situation though.

I am not sure if its worth caring for right now :)

Also the notifier chain will not start again until the previous call
chain is finished. So we are kind of synchronized here.

-- 
viresh

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#1736048

On 09/20, Viresh Kumar wrote:
> On 20-09-17, 10:00, Stephen Boyd wrote:
> > On 09/20, Viresh Kumar wrote:
> 
> > > diff --git a/drivers/base/power/opp/core.c b/drivers/base/power/opp/core.c
> > > index 4360b4efcd4c..668fd940d362 100644
> > > --- a/drivers/base/power/opp/core.c
> > > +++ b/drivers/base/power/opp/core.c
> > > @@ -1627,6 +1627,9 @@ static int _opp_set_availability(struct device *dev, unsigned long freq,
> > >  
> > >  	opp->available = availability_req;
> > >  
> > > +	dev_pm_opp_get(opp);
> > > +	mutex_unlock(&opp_table->lock);
> > 
> > Does this prevent the OPP from changing while the lock is
> > released?
> 
> No, its just ref counting and will only prevent it from getting freed.
> 
> There is only one thing that can change for an OPP though after it is
> created, its availability.

Ok.

> 
> > That would be the only difference from before. It's
> > possible that nobody cares about this situation though.
> 
> I am not sure if its worth caring for right now :)
> 
> Also the notifier chain will not start again until the previous call
> chain is finished. So we are kind of synchronized here.
> 

Yep. Just me worrying out loud. You can add my

Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>

-- 
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
a Linux Foundation Collaborative Project

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#1735928

On 09/20, Viresh Kumar wrote:
> The notifier callbacks may want to call some OPP helper routines which
> may try to take the same opp_table->lock again and cause a deadlock. One
> such usecase was reported by Chanwoo Choi, where calling
> dev_pm_opp_disable() leads us to the devfreq's OPP notifier handler,
> which further calls dev_pm_opp_find_freq_floor() and it deadlocks.
> 
> We don't really need the opp_table->lock to be held across the notifier
> call though, all we want to make sure is that the 'opp' doesn't get
> freed while being used from within the notifier chain. We can do it with
> help of dev_pm_opp_get/put() as well. Lets do it.

s/Lets/Let's/

> 
> Reported-by: Chanwoo Choi <cw00.choi@samsung.com>
> Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
> ---
>  drivers/base/power/opp/core.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/drivers/base/power/opp/core.c b/drivers/base/power/opp/core.c
> index 4360b4efcd4c..668fd940d362 100644
> --- a/drivers/base/power/opp/core.c
> +++ b/drivers/base/power/opp/core.c
> @@ -1627,6 +1627,9 @@ static int _opp_set_availability(struct device *dev, unsigned long freq,
>  
>  	opp->available = availability_req;
>  
> +	dev_pm_opp_get(opp);
> +	mutex_unlock(&opp_table->lock);

Does this prevent the OPP from changing while the lock is
released? That would be the only difference from before. It's
possible that nobody cares about this situation though.

-- 
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
a Linux Foundation Collaborative Project

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