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: 955ef4833574 (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>

diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
index e172b2a02c1d456df7bd51010a2074ea903a3735..e92e9eab7c6c9e32b5d85a73d1d053814a30dc6a 100644 (file)
--- a/drivers/cpufreq/cpufreq.c
+++ b/drivers/cpufreq/cpufreq.c
@@ -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 */

diff --git a/include/linux/cpufreq.h b/include/linux/cpufreq.h
index 704d85bf72423f9149ead4e549d4cb758d502d03..cac3d1ba82002c3bb9f0ffa6955cf7825fc52645 100644 (file)
--- a/include/linux/cpufreq.h
+++ b/include/linux/cpufreq.h
@@ -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;