commit 57714d5b1778f2f610bcc5c74d85b29ba1cc1995
Author: Viresh Kumar <viresh.kumar@linaro.org>
Date:   Wed Jan 21 11:31:39 2015 +0530

    cpufreq: Access governor's sysfs attributes without 'policy->rwsem'

    Commit 955ef4833574 ("cpufreq: Drop rwsem lock around CPUFREQ_GOV_POLICY_EXIT")
    dropped policy->rwsem because of some ABBA deadlocks.

    We weren't clear about the reasoning behind the deadlocks earlier and so dropped
    the locks before calling CPUFREQ_GOV_POLICY_EXIT.

    The actual problem was:

    If we hold any locks, that the attribute operations grab, when removing the
    attribute, then it can result in a ABBA deadlock.

    show()/store() holds the policy->rwsem lock while accessing any sysfs attributes
    under cpu/cpuX/cpufreq/ directory.

    Consider this scenario for drivers with CPUFREQ_HAVE_GOVERNOR_PER_POLICY flag:
    - Access any governor sysfs attribute. This will increment the active-count of
      sysfs attribute and show/store() of cpufreq core will get called. We will try
      to take policy->rwsem lock there, but following happens before that.

    - Another thread tries to change the governor by writing to
      cpuX/cpufreq/scaling_governor. We took policy->rwsem lock from store() and
      called CPUFREQ_GOV_POLICY_EXIT for governor.

      We call sysfs_remove_group() for the governor attributes, but that will wait
      for active-count to become zero.

    Now the first thread is waiting for the policy->rwsem lock and the next one for
    zero active-count.

    Perfect ABBA deadlock.

    Proposed solution: Accessing govenors attributes doesn't require policy->rwsem
    lock and so avoid it.

    - There are two code paths while accessing governor sysfs attributes.
      - Gov-per-policy: show/store() routines are called.
      - Common-gov: governor show/store routines called directly. In this case the
        governor might be getting used for multiple policies and so we couldn't take
        policy->rwsem lock there.

    So we need to avoid the locks only for the first case.

    This patch adds another field 'skip_lock' to 'struct freq_attr' and is set only
    for the governors. When this is set, we don't take policy->rwsem lock in
    show/store() routines.

    Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>