Commit 68e80dae authored Feb 09, 2016 by Viresh Kumar Committed by Rafael J. Wysocki Mar 09, 2016

Revert "cpufreq: Drop rwsem lock around CPUFREQ_GOV_POLICY_EXIT"

Earlier, when the struct freq-attr was used to represent governor
attributes, the standard cpufreq show/store sysfs attribute callbacks
were applied to the governor tunable attributes and they always acquire
the policy->rwsem lock before carrying out the operation. That could
have resulted in an ABBA deadlock if governor tunable attributes are
removed under policy->rwsem while one of them is being accessed
concurrently (if sysfs attributes removal wins the race, it will wait
for the access to complete with policy->rwsem held while the attribute
callback will block on policy->rwsem indefinitely).

We attempted to address this issue by dropping policy->rwsem around
governor tunable attributes removal (that is, around invocations of the
->governor callback with the event arg equal to CPUFREQ_GOV_POLICY_EXIT)
in cpufreq_set_policy(), but that opened up race conditions that had not
been possible with policy->rwsem held all the time.

The previous commit, "cpufreq: governor: New sysfs show/store callbacks
for governor tunables", fixed the original ABBA deadlock by adding new
governor specific show/store callbacks.

We don't have to drop rwsem around invocations of governor event
CPUFREQ_GOV_POLICY_EXIT anymore, and original fix can be reverted now.

Fixes: 955ef483 (cpufreq: Drop rwsem lock around CPUFREQ_GOV_POLICY_EXIT)
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reported-by: Juri Lelli <juri.lelli@arm.com>
Tested-by: Juri Lelli <juri.lelli@arm.com>
Tested-by: Shilpasri G Bhat <shilpa.bhat@linux.vnet.ibm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

parent fd8ddc48

drivers/cpufreq/cpufreq.c

+0 −5

Original line number	Diff line number	Diff line
		@@ -2205,10 +2205,7 @@ static int cpufreq_set_policy(struct cpufreq_policy *policy,
		return ret;
		}

		up_write(&policy->rwsem);
		ret = __cpufreq_governor(policy, CPUFREQ_GOV_POLICY_EXIT);
		down_write(&policy->rwsem);

		if (ret) {
		pr_err("%s: Failed to Exit Governor: %s (%d)\n",
		__func__, old_gov->name, ret);
		@@ -2224,9 +2221,7 @@ static int cpufreq_set_policy(struct cpufreq_policy *policy,
		if (!ret)
		goto out;

		up_write(&policy->rwsem);
		__cpufreq_governor(policy, CPUFREQ_GOV_POLICY_EXIT);
		down_write(&policy->rwsem);
		}

		/* new governor failed, so re-start old one */

include/linux/cpufreq.h

+0 −4

Original line number	Diff line number	Diff line
		@@ -100,10 +100,6 @@ struct cpufreq_policy {
		* - Any routine that will write to the policy structure and/or may take away
		* the policy altogether (eg. CPU hotplug), will hold this lock in write
		* mode before doing so.
		*
		* Additional rules:
		* - Lock should not be held across
		* __cpufreq_governor(data, CPUFREQ_GOV_POLICY_EXIT);
		*/
		struct rw_semaphore rwsem;

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