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[PATCH V2] PM / OPP: Call notifier without holding opp_table->lock

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  [PATCH V2] PM / OPP: Call notifier without holding opp_table->lock Viresh Kumar <viresh.kumar@linaro.org> - 2017-09-20 22:30 +0200
    Re: [PATCH V2] PM / OPP: Call notifier without holding  opp_table->lock Chanwoo Choi <cw00.choi@samsung.com> - 2017-09-21 02:00 +0200

#1736083 — [PATCH V2] 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. Let's do it.

Reported-by: Chanwoo Choi <cw00.choi@samsung.com>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
---
V1->V2:
- s/Lets/Let's/ in commit log and added Stephen's tag.

 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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#1736271 — Re: [PATCH V2] PM / OPP: Call notifier without holding opp_table->lock

Hi Viresh,

On 2017년 09월 21일 05:25, 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. Let's do it.
> 
> Reported-by: Chanwoo Choi <cw00.choi@samsung.com>
> Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
> Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
> ---
> V1->V2:
> - s/Lets/Let's/ in commit log and added Stephen's tag.
> 

Thanks for your fixup. Looks good to me.
IMHO, this patch should be posted to stable@vger.kernel.org.

Tested-by: Chanwoo Choi <cw00.choi@samsung.com>
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>

[snip] 

-- 
Best Regards,
Chanwoo Choi
Samsung Electronics

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