drm/i915: Mark i915_request.timeline as a volatile, rcu pointer

The request->timeline is only valid until the request is retired (i.e.
before it is completed). Upon retiring the request, the context may be
unpinned and freed, and along with it the timeline may be freed. We
therefore need to be very careful when chasing rq->timeline that the
pointer does not disappear beneath us. The vast majority of users are in
a protected context, either during request construction or retirement,
where the timeline->mutex is held and the timeline cannot disappear. It
is those few off the beaten path (where we access a second timeline) that
need extra scrutiny -- to be added in the next patch after first adding
the warnings about dangerous access.

One complication, where we cannot use the timeline->mutex itself, is
during request submission onto hardware (under spinlocks). Here, we want
to check on the timeline to finalize the breadcrumb, and so we need to
impose a second rule to ensure that the request->timeline is indeed
valid. As we are submitting the request, it's context and timeline must
be pinned, as it will be used by the hardware. Since it is pinned, we
know the request->timeline must still be valid, and we cannot submit the
idle barrier until after we release the engine->active.lock, ergo while
submitting and holding that spinlock, a second thread cannot release the
timeline.

v2: Don't be lazy inside selftests; hold the timeline->mutex for as long
as we need it, and tidy up acquiring the timeline with a bit of
refactoring (i915_active_add_request)

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190919111912.21631-1-chris@chris-wilson.co.uk

1 files changed, 21 insertions, 1 deletions

diff --git a/drivers/gpu/drm/i915/i915_request.h b/drivers/gpu/drm/i915/i915_request.h
index 8ac6e1226a56..b18f49528ded 100644
--- a/drivers/gpu/drm/i915/i915_request.h
+++ b/drivers/gpu/drm/i915/i915_request.h
@@ -113,7 +113,7 @@ struct i915_request {
 	struct intel_engine_cs *engine;
 	struct intel_context *hw_context;
 	struct intel_ring *ring;
-	struct intel_timeline *timeline;
+	struct intel_timeline __rcu *timeline;
 	struct list_head signal_link;
 
 	/*
@@ -442,6 +442,26 @@ static inline bool i915_request_has_nopreempt(const struct i915_request *rq)
 	return unlikely(rq->flags & I915_REQUEST_NOPREEMPT);
 }
 
+static inline struct intel_timeline *
+i915_request_timeline(struct i915_request *rq)
+{
+	/* Valid only while the request is being constructed (or retired). */
+	return rcu_dereference_protected(rq->timeline,
+					 lockdep_is_held(&rcu_access_pointer(rq->timeline)->mutex));
+}
+
+static inline struct intel_timeline *
+i915_request_active_timeline(struct i915_request *rq)
+{
+	/*
+	 * When in use during submission, we are protected by a guarantee that
+	 * the context/timeline is pinned and must remain pinned until after
+	 * this submission.
+	 */
+	return rcu_dereference_protected(rq->timeline,
+					 lockdep_is_held(&rq->engine->active.lock));
+}
+
 bool i915_retire_requests(struct drm_i915_private *i915);
 
 #endif /* I915_REQUEST_H */