[Intel-gfx] [PATCH v2] drm/i915: Lock signaler timeline while navigating

[Chris Wilson]

As we need to take a walk back along the signaler timeline to find the
fence before upon which we want to wait, we need to lock that timeline
to prevent it being modified as we walk. Similarly, we also need to
acquire a reference to the earlier fence while it still exists!

Though we lack the correct locking today, we are saved by the
overarching struct_mutex -- but that protection is being removed.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin at linux.intel.com>
---
Add the lockdep assert and GEM_BUG_ON to note the overlapping timelines.
---
 drivers/gpu/drm/i915/i915_request.c | 31 +++++++++++++++++++++++------
 1 file changed, 25 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
index f12358150097..3966b330b5de 100644
--- a/drivers/gpu/drm/i915/i915_request.c
+++ b/drivers/gpu/drm/i915/i915_request.c
@@ -767,16 +767,35 @@ i915_request_create(struct intel_context *ce)
 static int
 i915_request_await_start(struct i915_request *rq, struct i915_request *signal)
 {
-	if (list_is_first(&signal->link, &signal->timeline->requests))
+	struct intel_timeline *tl = signal->timeline;
+	struct dma_fence *fence;
+	int err;
+
+	lockdep_assert_held(&rq->timeline->mutex);
+	GEM_BUG_ON(rq->timeline == signal->timeline);
+
+	if (list_is_first(&signal->link, &tl->requests))
 		return 0;
 
-	signal = list_prev_entry(signal, link);
-	if (intel_timeline_sync_is_later(rq->timeline, &signal->fence))
+	if (mutex_lock_interruptible_nested(&tl->mutex, SINGLE_DEPTH_NESTING))
+		return -EINTR;
+
+	fence = NULL;
+	if (!list_is_first(&signal->link, &tl->requests))
+		fence = dma_fence_get(&list_prev_entry(signal, link)->fence);
+
+	mutex_unlock(&tl->mutex);
+	if (!fence)
 		return 0;
 
-	return i915_sw_fence_await_dma_fence(&rq->submit,
-					     &signal->fence, 0,
-					     I915_FENCE_GFP);
+	err = 0;
+	if (!intel_timeline_sync_is_later(rq->timeline, fence))
+		err = i915_sw_fence_await_dma_fence(&rq->submit,
+						    fence, 0,
+						    I915_FENCE_GFP);
+	dma_fence_put(fence);
+
+	return err;
 }
 
 static intel_engine_mask_t
-- 
2.23.0