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    Subject[PATCH V2 4/7] Revert "cpufreq: Drop rwsem lock around CPUFREQ_GOV_POLICY_EXIT"
    Date
    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")
    Reported-by: Juri Lelli <juri.lelli@arm.com>
    Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
    ---
    drivers/cpufreq/cpufreq.c | 5 -----
    include/linux/cpufreq.h | 4 ----
    2 files changed, 9 deletions(-)

    diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
    index e979ec78b695..5f7e24567e0e 100644
    --- a/drivers/cpufreq/cpufreq.c
    +++ b/drivers/cpufreq/cpufreq.c
    @@ -2155,10 +2155,7 @@ 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);
    @@ -2174,9 +2171,7 @@ 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 88a4215125bc..79b87cebaa9c 100644
    --- 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;

    --
    2.7.0.79.gdc08a19
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