drm/i915/gt: Restore ce->signal flush before releasing virtual engine

Before we mark the virtual engine as no longer inflight, flush any
ongoing signaling that may be using the ce->signal_link along the
previous breadcrumbs. On switch to a new physical engine, that link will
be inserted into the new set of breadcrumbs, causing confusion to an
ongoing iterator.

This patch undoes a last minute mistake introduced into commit
bab0557c8d ("drm/i915/gt: Remove virtual breadcrumb before transfer"),
whereby instead of unconditionally applying the flush, it was only
applied if the request itself was going to be reused.

v2: Generalise and cancel all remaining ce->signals

Fixes: bab0557c8d ("drm/i915/gt: Remove virtual breadcrumb before transfer")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210108204026.20682-4-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gt/intel_breadcrumbs.c

@@ -472,6 +472,39 @@ void i915_request_cancel_breadcrumb(struct i915_request *rq)
 	i915_request_put(rq);
 }
 
+void intel_context_remove_breadcrumbs(struct intel_context *ce,
+				      struct intel_breadcrumbs *b)
+{
+	struct i915_request *rq, *rn;
+	bool release = false;
+	unsigned long flags;
+
+	spin_lock_irqsave(&ce->signal_lock, flags);
+
+	if (list_empty(&ce->signals))
+		goto unlock;
+
+	list_for_each_entry_safe(rq, rn, &ce->signals, signal_link) {
+		GEM_BUG_ON(!__i915_request_is_complete(rq));
+		if (!test_and_clear_bit(I915_FENCE_FLAG_SIGNAL,
+					&rq->fence.flags))
+			continue;
+
+		list_del_rcu(&rq->signal_link);
+		irq_signal_request(rq, b);
+		i915_request_put(rq);
+	}
+	release = remove_signaling_context(b, ce);
+
+unlock:
+	spin_unlock_irqrestore(&ce->signal_lock, flags);
+	if (release)
+		intel_context_put(ce);
+
+	while (atomic_read(&b->signaler_active))
+		cpu_relax();
+}
+
 static void print_signals(struct intel_breadcrumbs *b, struct drm_printer *p)
 {
 	struct intel_context *ce;

drivers/gpu/drm/i915/gt/intel_breadcrumbs.h

@@ -6,6 +6,7 @@
 #ifndef __INTEL_BREADCRUMBS__
 #define __INTEL_BREADCRUMBS__
 
+#include <linux/atomic.h>
 #include <linux/irq_work.h>
 
 #include "intel_engine_types.h"
@@ -44,4 +45,7 @@ void intel_engine_print_breadcrumbs(struct intel_engine_cs *engine,
 bool i915_request_enable_breadcrumb(struct i915_request *request);
 void i915_request_cancel_breadcrumb(struct i915_request *request);
 
+void intel_context_remove_breadcrumbs(struct intel_context *ce,
+				      struct intel_breadcrumbs *b);
+
 #endif /* __INTEL_BREADCRUMBS__ */

drivers/gpu/drm/i915/gt/intel_execlists_submission.c

@@ -581,21 +581,6 @@ resubmit_virtual_request(struct i915_request *rq, struct virtual_engine *ve)
 {
 	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);
-		while (atomic_read(&engine->breadcrumbs->signaler_active))
-			cpu_relax();
-	}
-
 	spin_lock_irq(&engine->active.lock);
 
 	clear_bit(I915_FENCE_FLAG_PQUEUE, &rq->fence.flags);
@@ -610,6 +595,16 @@ 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;
 
+	/*
+	 * 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.
+	 */
+	if (!list_empty(&ce->signals))
+		intel_context_remove_breadcrumbs(ce, engine->breadcrumbs);
+
 	/*
 	 * This engine is now too busy to run this virtual request, so
 	 * see if we can find an alternative engine for it to execute on.