[PATCH v4 0/4] devfreq: Add refcounts for governor modules

A governor module can be dynamically inserted or removed if compiled as a
kernel module.  'devfreq->governor' would become NULL if the governor
module is removed when it's in use.

For user-friendliness, get and put the module refcount of a governor module
when it's in use.  As a result, unloading a governor module in use returns
an error, e.g.:

$ cat governor
performance
$ rmmod governor_performance
rmmod: ERROR: Module governor_performance is in use

Note that this can't stop force unload, so it's more of a
user-friendliness improvement rather than strict protection.  The
existing code that keeps devfreq working when 'devfreq->governor' is
NULL should still be there.

Patch 1-3 clean up mutex with guards and factor out a common governor
setting function, so as to prepare for implementing governor reference
counting.  They can be applied separately.

Patch 4 adds the reference counting mechanism for devfreq governor
modules.

This set is based on devfreq-next of 7.1-rc1.

Changelog
---------
v4:
- Patch 3: Return an error if starting new governor fails and restoring
  old governor succeeds, such that the caller won't get misled.

v3:
- Link: https://lore.kernel.org/all/[email protected]/
- Rework patch 4 to make the 'owner' field assignment optional in the
  governor code.  This prevents device-driver-bundled governors (such as
  those in hisi_uncore_freq and tegra30-devfreq) from causing
  self-reference issues that block module removal. Additionally, merge
  patches 4 and 5 to clearly present the new 'owner' member alongside
  its usage.
- Drop patch 6 because it's no longer needed since v2.
- Update commit logs of patch 3 and 4 to clarify background and
  motivation.
- Pick up 2 tags from Yaxiong (thanks!).
- Trivial cleanups.

v2:
- Link: https://lore.kernel.org/all/[email protected]/
- Rebase on devfreq-next of 7.1-rc1.
- Drop the patches related to the NULL pointer deference issue of
  'devfreq->governor', which has been solved and merged recently.
- Remove the dedicated mutex for 'devfreq_governor_list' because the
  refcount changes don't depend on that.
- Some minor cleanups and fixes.

v1:
- Link: https://lore.kernel.org/all/[email protected]/

Jie Zhan (4):
  devfreq: Use mutex guard in governor_store()
  devfreq: Use mutex guard in devfreq_add/remove_governor()
  devfreq: Factor out devfreq_set_governor()
  devfreq: Refcount governor modules while in use

 drivers/devfreq/devfreq.c                 | 185 +++++++++++-----------
 drivers/devfreq/governor_passive.c        |   1 +
 drivers/devfreq/governor_performance.c    |   1 +
 drivers/devfreq/governor_powersave.c      |   1 +
 drivers/devfreq/governor_simpleondemand.c |   1 +
 drivers/devfreq/governor_userspace.c      |   1 +
 include/linux/devfreq-governor.h          |  11 ++
 7 files changed, 112 insertions(+), 89 deletions(-)

-- 
2.43.0