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

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From	Viresh Kumar <viresh.kumar@linaro.org>
Newsgroups	linux.kernel
Subject	[PATCH] PM / OPP: Call notifier without holding opp_table->lock
Date	Wed, 20 Sep 2017 17:40:02 +0200
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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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[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

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