[PATCH 27/29] drm/i915/gt: Remove virtual breadcrumb before transfer

[Chris Wilson]

The issue with stale virtual breadcrumbs remain. Now we have the problem
that if the irq-signaler is still referencing the stale breadcrumb as we
transfer it to a new sibling, the list becomes spaghetti. This is a very
small window, but that doesn't stop it being hit infrequently. To
prevent the lists being tangled (the iterator starting on one engine's
b->signalers but walking onto another list), always decouple the virtual
breadcrumb on schedule-out and make sure that the walker has stepped out
of the lists.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/gt/intel_breadcrumbs.c |  5 +++--
 drivers/gpu/drm/i915/gt/intel_lrc.c         | 15 +++++++++++++++
 2 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c b/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
index f5f6feed0fa6..5b84f51931d9 100644
--- a/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
+++ b/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
@@ -461,15 +461,16 @@ void i915_request_cancel_breadcrumb(struct i915_request *rq)
 {
 	struct intel_breadcrumbs *b = READ_ONCE(rq->engine)->breadcrumbs;
 	struct intel_context *ce = rq->context;
+	unsigned long flags;
 	bool release;
 
 	if (!test_and_clear_bit(I915_FENCE_FLAG_SIGNAL, &rq->fence.flags))
 		return;
 
-	spin_lock(&ce->signal_lock);
+	spin_lock_irqsave(&ce->signal_lock, flags);
 	list_del_rcu(&rq->signal_link);
 	release = remove_signaling_context(b, ce);
-	spin_unlock(&ce->signal_lock);
+	spin_unlock_irqrestore(&ce->signal_lock, flags);
 	if (release)
 		intel_context_put(ce);
 
diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
index 3ec85d456195..47efc1d76550 100644
--- a/drivers/gpu/drm/i915/gt/intel_lrc.c
+++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
@@ -1387,6 +1387,21 @@ static inline void execlists_schedule_in(struct i915_request *rq, int idx)
 static void kick_siblings(struct i915_request *rq, struct intel_context *ce)
 {
 	struct virtual_engine *ve = container_of(ce, typeof(*ve), context);
+	struct intel_engine_cs *engine = rq->engine;
+
+	/* Flush concurrent rcu iterators in signal_irq_work */
+	if (test_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT, &rq->fence.flags)) {
+		/*
+		 * After this point, the rq may be transferred to a new
+		 * sibling, so before we clear ce->inflight make sure that
+		 * the context has been removed from the b->signalers and
+		 * furthermore we need to make sure that the concurrent
+		 * iterator in signal_irq_work is no longer following
+		 * ce->signal_link.
+		 */
+		i915_request_cancel_breadcrumb(rq);
+		irq_work_sync(&engine->breadcrumbs->irq_work);
+	}
 
 	if (READ_ONCE(ve->request))
 		tasklet_hi_schedule(&ve->base.execlists.tasklet);
-- 
2.20.1