[Intel-gfx] [PATCH] drm/i915: Prevent bonded requests from overtaking each other on preemption

[Chris Wilson]

Force bonded requests to run on distinct engines so that they cannot be
shuffled onto the same engine where timeslicing will reverse the order.
A bonded request will often wait on a semaphore signaled by its master,
creating an implicit dependency -- if we ignore that implicit dependency
and allow the bonded request to run on the same engine and before its
master, we will cause a GPU hang.

We can prevent this inversion by restricting which engines we allow
ourselves to jump to upon preemption, i.e. baking in the arrangement
established at first execution. (We should also consider capturing the
implicit dependency using i915_sched_add_dependency(), but first we need
to think about the constraints that requires on the execution/retirement
ordering.)

Fixes: 8ee36e048c98 ("drm/i915/execlists: Minimalistic timeslicing")
References: ee1136908e9b ("drm/i915/execlists: Virtual engine bonding")
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala at linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
---
 drivers/gpu/drm/i915/gt/intel_lrc.c | 19 +++++++++++--------
 1 file changed, 11 insertions(+), 8 deletions(-)

diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
index a99166a2d2eb..7920649e4d87 100644
--- a/drivers/gpu/drm/i915/gt/intel_lrc.c
+++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
@@ -3755,18 +3755,21 @@ static void
 virtual_bond_execute(struct i915_request *rq, struct dma_fence *signal)
 {
 	struct virtual_engine *ve = to_virtual_engine(rq->engine);
+	intel_engine_mask_t allowed, exec;
 	struct ve_bond *bond;
 
 	bond = virtual_find_bond(ve, to_request(signal)->engine);
-	if (bond) {
-		intel_engine_mask_t old, new, cmp;
+	if (!bond)
+		return;
 
-		cmp = READ_ONCE(rq->execution_mask);
-		do {
-			old = cmp;
-			new = cmp & bond->sibling_mask;
-		} while ((cmp = cmpxchg(&rq->execution_mask, old, new)) != old);
-	}
+	/* Restrict the bonded request to run on only the slaved engines */
+	allowed = bond->sibling_mask & ~to_request(signal)->engine->mask;
+	exec = READ_ONCE(rq->execution_mask);
+	while (!try_cmpxchg(&rq->execution_mask, &exec, exec & allowed))
+		;
+
+	/* Prevent the master from being re-run on the slaved engines */
+	to_request(signal)->execution_mask &= ~allowed;
 }
 
 struct intel_context *
-- 
2.23.0