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    • Dave Airlie's avatar
      Merge branch 'vmwgfx-next' of git://people.freedesktop.org/~thomash/linux into drm-next · c5be9b54
      Dave Airlie authored
      A patchset worked out together with Peter Zijlstra. Ingo is OK with taking
      it through the DRM tree:
      
      This is a small fallout from a work to allow batching WW mutex locks and
      unlocks.
      
      Our Wound-Wait mutexes actually don't use the Wound-Wait algorithm but
      the Wait-Die algorithm. One could perhaps rename those mutexes tree-wide to
      "Wait-Die mutexes" or "Deadlock Avoidance mutexes". Another approach suggested
      here is to implement also the "Wound-Wait" algorithm as a per-WW-class
      choice, as it has advantages in some cases. See for example
      
      http://www.mathcs.emory.edu/~cheung/Courses/554/Syllabus/8-recv+serial/deadlock-compare.html
      
      Now Wound-Wait is a preemptive algorithm, and the preemption is implemented
      using a lazy scheme: If a wounded transaction is about to go to sleep on
      a contended WW mutex, we return -EDEADLK. That is sufficient for deadlock
      prevention. Since with WW mutexes we also require the aborted transaction to
      sleep waiting to lock the WW mutex it was aborted on, this choice also provides
      a suitable WW mutex to sleep on. If we were to return -EDEADLK on the first
      WW mutex lock after the transaction was wounded whether the WW mutex was
      contended or not, the transaction might frequently be restarted without a wait,
      which is far from optimal. Note also that with the lazy preemption scheme,
      contrary to Wait-Die there will be no rollbacks on lock contention of locks
      held by a transaction that has completed its locking sequence.
      
      The modeset locks are then changed from Wait-Die to Wound-Wait since the
      typical locking pattern of those locks very well matches the criterion for
      a substantial reduction in the number of rollbacks. For reservation objects,
      the benefit is more unclear at this point and they remain using Wait-Die.
      Signed-off-by: default avatarDave Airlie <airlied@redhat.com>
      Link: https://patchwork.freedesktop.org/patch/msgid/20180703105339.4461-1-thellstrom@vmware.com
      c5be9b54