drm/i915/gt: Shrink the critical section for irq signaling

Let's only wait for the list iterator when decoupling the virtual
breadcrumb, as the signaling of all the requests may take a long time,
during which we do not want to keep the tasklet spinning.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201224135544.1713-6-chris@chris-wilson.co.uk

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

@@ -234,6 +234,7 @@ static void signal_irq_work(struct irq_work *work)
 		intel_breadcrumbs_disarm_irq(b);
 
 	rcu_read_lock();
+	atomic_inc(&b->signaler_active);
 	list_for_each_entry_rcu(ce, &b->signalers, signal_link) {
 		struct i915_request *rq;
 
@@ -269,6 +270,7 @@ static void signal_irq_work(struct irq_work *work)
 			}
 		}
 	}
+	atomic_dec(&b->signaler_active);
 	rcu_read_unlock();
 
 	llist_for_each_safe(signal, sn, signal) {

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

@@ -34,6 +34,7 @@ struct intel_breadcrumbs {
 	spinlock_t signalers_lock; /* protects the list of signalers */
 	struct list_head signalers;
 	struct llist_head signaled_requests;
+	atomic_t signaler_active;
 
 	spinlock_t irq_lock; /* protects the interrupt from hardirq context */
 	struct irq_work irq_work; /* for use from inside irq_lock */

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

@@ -594,7 +594,8 @@ static void kick_siblings(struct i915_request *rq, struct intel_context *ce)
 		 * ce->signal_link.
 		 */
 		i915_request_cancel_breadcrumb(rq);
-		irq_work_sync(&engine->breadcrumbs->irq_work);
+		while (atomic_read(&engine->breadcrumbs->signaler_active))
+			cpu_relax();
 	}
 
 	if (READ_ONCE(ve->request))