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1255501d86
Daniel Vetter proposed a new challenge to the serialisation inside the busy-ioctl that exposed a flaw that could result in us reporting the wrong engine as being busy. If the request is reallocated as we test its busyness and then reassigned to this object by another thread, we would not notice that the test itself was incorrect. We are faced with a choice of using __i915_gem_active_get_request_rcu() to first acquire a reference to the request preventing the race, or to acknowledge the race and accept the limitations upon the accuracy of the busy flags. Note that we guarantee that we never falsely report the object as idle (providing userspace itself doesn't race), and so the most important use of the busy-ioctl and its guarantees are fulfilled. Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk> Cc: Daniel Vetter <daniel.vetter@intel.com> Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com> Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com> Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com> Reviewed-by: Daniel Vetter <daniel.vetter@intel.com> Link: http://patchwork.freedesktop.org/patch/msgid/1471337440-16777-1-git-send-email-chris@chris-wilson.co.uk |
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