[Intel-gfx] [PATCH] drm/i915: Check against the signaled bit for fences/requests

[Chris Wilson]

When dma_fence_signal() is called, it sets a flag to indicate the fenece
is complete. Before the dma_fence is signaled, the seqno check will
first be passed. However, if the request is being held by an observer
across a global seqno reset, it is possible for the observer to not
notice the seqno completion (as the HWS is now zero), but instead we
rely on the signaled bit. External holders of the request should be
using the fence interface which does first check the signaled bit - it
will only be an unlocked waiter that may not notice the request
completion.

By that analysis, the problem is old as our unlocked waits, but this
patch depends upon subclassing dma-fence, so use that as the base
commit.

Fixes: 04769652c8c7 ("drm/i915: Derive GEM requests from dma-fence")
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin at linux.intel.com>
Cc: <stable at vger.kernel.org> # v4.9+
---
 drivers/gpu/drm/i915/i915_drv.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 994929660027..dcc8f73b26bf 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -4026,6 +4026,9 @@ __i915_request_irq_complete(struct drm_i915_gem_request *req)
 {
 	struct intel_engine_cs *engine = req->engine;
 
+	if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &req->fence.flags))
+		return true;
+
 	/* Before we do the heavier coherent read of the seqno,
 	 * check the value (hopefully) in the CPU cacheline.
 	 */
-- 
2.11.0