[0/2] cpufreq: Fix a circular lock dependency problem

Message

Waiman Long July 23, 2018, 5:49 p.m. UTC

This patchset works around a circular lock dependency issue in the
cpufreq driver reported by lockdep. The two locks involved are the
cpu_hotplup_lock and the reference count of a sysfs file.

The cpufreq_register_driver() function uses the lock sequence:

  cpus_read_lock --> kn->count

Whereas the cpufreq sysfs store method uses the sequence:

  kn->count --> cpus_read_lock

This is not really an issue as a shared lock is used on the
cpu_hotplup_lock. However, the lockdep code isn't able to handle
shared locking. So one way to work around this is to define a
cpus_read_trylock() function and uses it in the store method instead.

Waiman Long (2):
  cpu/hotplug: Add a cpus_read_trylock() function
  cpufreq: Fix a circular lock dependency problem

 drivers/cpufreq/cpufreq.c | 16 +++++++++++++++-
 include/linux/cpu.h       |  2 ++
 kernel/cpu.c              |  6 ++++++
 3 files changed, 23 insertions(+), 1 deletion(-)