drm/i915/gt: Close race between engine_park and intel_gt_retire_requests
authorChris Wilson <chris@chris-wilson.co.uk>
Wed, 20 Nov 2019 16:55:13 +0000 (16:55 +0000)
committerChris Wilson <chris@chris-wilson.co.uk>
Wed, 20 Nov 2019 16:57:33 +0000 (16:57 +0000)
commita6edbca74b305adc165e67065d7ee766006e6a48
treea816acebf1d4c24be2deba6fdc827812f9a22cbe
parent07779a76ee1f93f930cf697b22be73d16e14f50c
drm/i915/gt: Close race between engine_park and intel_gt_retire_requests

The general concept was that intel_timeline.active_count was locked by
the intel_timeline.mutex. The exception was for power management, where
the engine->kernel_context->timeline could be manipulated under the
global wakeref.mutex.

This was quite solid, as we always manipulated the timeline only while
we held an engine wakeref.

And then we started retiring requests outside of struct_mutex, only
using the timelines.active_list and the timeline->mutex. There we
started manipulating intel_timeline.active_count outside of an engine
wakeref, and so introduced a race between __engine_park() and
intel_gt_retire_requests(), a race that could result in the
engine->kernel_context not being added to the active timelines and so
losing requests, which caused us to keep the system permanently powered
up [and unloadable].

The race would be easy to close if we could take the engine wakeref for
the timeline before we retire -- except timelines are not bound to any
engine and so we would need to keep all active engines awake. The
alternative is to guard intel_timeline_enter/intel_timeline_exit for use
outside of the timeline->mutex.

Fixes: e5dadff4b093 ("drm/i915: Protect request retirement with timeline->mutex")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191120165514.3955081-1-chris@chris-wilson.co.uk
drivers/gpu/drm/i915/gt/intel_gt_requests.c
drivers/gpu/drm/i915/gt/intel_timeline.c
drivers/gpu/drm/i915/gt/intel_timeline_types.h