[PATCH v4 0/5] drm: update locking for modesetting

Desmond Cheong Zhi Xi desmondcheongzx at gmail.com
Fri Aug 20 10:02:46 UTC 2021


Thanks for all the helpful feedback on the previous version.

Taking all the suggestions together, this series now converts
drm_device.master_mutex into master_rwsem, and also attempts to remove
drm_file.master_lookup_lock. There might still be lock inversions
lurking, so the output from Intel-gfx CI should be interesting to see.

Overall, this series makes the following changes:

- Patch 1: Fix a potential null ptr dereference in drm_master_release

- Patch 2: Convert master_mutex into rwsem (avoids creating a new lock)

- Patch 3: Update global mutex locking in the ioctl handler (avoids
deadlock when grabbing read lock on master_rwsem in drm_ioctl_kernel)

- Patch 4: Plug races with drm modesetting rights

- Patch 5: Replace master_lookup_lock with master_rwsem by untangling
remaining lock hierarchy inversions

v3 -> v4 (suggested by Daniel Vetter):
- Drop a patch that added an unnecessary master_lookup_lock in
drm_master_release (previously patch 2)
- Drop a patch that addressed a non-existent race in
drm_is_current_master_locked (previously patch 3)
- Remove fixes for non-existent null ptr dereferences (previous patch 4)
- Protect drm_master.magic_map,unique{_len} with master_rwsem instead of
master_lookup_lock (dropped previous patch 5)
- Drop the patch that moved master_lookup_lock into struct drm_device
(previously patch 1)
- Drop a patch to export task_work_add (previously patch 8)
- Revert the check for the global mutex in the ioctl handler to use
drm_core_check_feature instead of drm_dev_needs_global_mutex
- Push down master_rwsem locking for selected ioctls to avoid lock
hierarchy inversions, and to allow us to hold write locks on
master_rwsem instead of flushing readers
- Remove master_lookup_lock by replacing it with master_rwsem

v2 -> v3:
- Unexport drm_master_flush, as suggested by Daniel Vetter.
- Merge master_mutex and master_rwsem, as suggested by Daniel Vetter.
- Export task_work_add, reported by kernel test robot.
- Make master_flush static, reported by kernel test robot.
- Move master_lookup_lock into struct drm_device.
- Add a missing lock on master_lookup_lock in drm_master_release.
- Fix a potential race in drm_is_current_master_locked.
- Fix potential null ptr dereferences in drm_{auth, ioctl}.
- Protect magic_map,unique{_len} with  master_lookup_lock.
- Convert master_mutex into a rwsem.
- Update global mutex locking in the ioctl handler.

v1 -> v2 (suggested by Daniel Vetter):
- Address an additional race when drm_open runs.
- Switch from SRCU to rwsem to synchronise readers and writers.
- Implement drm_master_flush with task_work so that flushes can be
queued to run before returning to userspace without creating a new
DRM_MASTER_FLUSH ioctl flag.

Best wishes,

Desmond Cheong Zhi Xi (5):
  drm: fix null ptr dereference in drm_master_release
  drm: convert drm_device.master_mutex into a rwsem
  drm: lock drm_global_mutex earlier in the ioctl handler
  drm: avoid races with modesetting rights
  drm: remove drm_file.master_lookup_lock

 drivers/gpu/drm/drm_auth.c        | 54 ++++++++++++------------
 drivers/gpu/drm/drm_debugfs.c     |  4 +-
 drivers/gpu/drm/drm_drv.c         |  3 +-
 drivers/gpu/drm/drm_file.c        |  7 ++--
 drivers/gpu/drm/drm_internal.h    |  1 +
 drivers/gpu/drm/drm_ioctl.c       | 48 ++++++++++++---------
 drivers/gpu/drm/drm_lease.c       | 69 ++++++++++++++-----------------
 drivers/gpu/drm/drm_mode_object.c | 14 +++++--
 include/drm/drm_auth.h            |  6 +--
 include/drm/drm_device.h          | 15 +++++--
 include/drm/drm_file.h            | 17 +++-----
 include/drm/drm_lease.h           |  2 +-
 12 files changed, 125 insertions(+), 115 deletions(-)


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