[PATCH 14/37] drm/i915: Take a reference whilst processing the signaler request

[Chris Wilson]

The plan in the near-future is to allow requests to be removed from the
signaler. We can no longer then rely on holding a reference to the
request for the duration it is in the signaling tree, and instead must
obtain a reference to the request for the current operation using RCU.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/intel_breadcrumbs.c | 22 +++++++++++++++-------
 drivers/gpu/drm/i915/intel_ringbuffer.h  |  2 +-
 2 files changed, 16 insertions(+), 8 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_breadcrumbs.c b/drivers/gpu/drm/i915/intel_breadcrumbs.c
index e1d9ad30b978..201f138d753c 100644
--- a/drivers/gpu/drm/i915/intel_breadcrumbs.c
+++ b/drivers/gpu/drm/i915/intel_breadcrumbs.c
@@ -467,7 +467,11 @@ static int intel_breadcrumbs_signaler(void *arg)
 		 * need to wait for a new interrupt from the GPU or for
 		 * a new client.
 		 */
-		request = READ_ONCE(b->first_signal);
+		rcu_read_lock();
+		request = rcu_dereference(b->first_signal);
+		if (request)
+			request = i915_gem_request_get_rcu(request);
+		rcu_read_unlock();
 		if (signal_complete(request)) {
 			/* Wake up all other completed waiters and select the
 			 * next bottom-half for the next user interrupt.
@@ -486,21 +490,25 @@ static int intel_breadcrumbs_signaler(void *arg)
 			 * the oldest before picking the next one.
 			 */
 			spin_lock_irq(&b->lock);
-			if (request == b->first_signal) {
+			if (request == rcu_access_pointer(b->first_signal)) {
 				struct rb_node *rb =
 					rb_next(&request->signaling.node);
-				b->first_signal = rb ? to_signaler(rb) : NULL;
+				rcu_assign_pointer(b->first_signal,
+						   rb ? to_signaler(rb) : NULL);
 			}
 			rb_erase(&request->signaling.node, &b->signals);
 			spin_unlock_irq(&b->lock);
 
 			i915_gem_request_put(request);
 		} else {
-			if (kthread_should_stop())
+			if (kthread_should_stop()) {
+				GEM_BUG_ON(request);
 				break;
+			}
 
 			schedule();
 		}
+		i915_gem_request_put(request);
 	} while (1);
 	__set_current_state(TASK_RUNNING);
 
@@ -636,7 +644,7 @@ void intel_engine_fini_breadcrumbs(struct intel_engine_cs *engine)
 	/* The engines should be idle and all requests accounted for! */
 	WARN_ON(READ_ONCE(b->first_wait));
 	WARN_ON(!RB_EMPTY_ROOT(&b->waiters));
-	WARN_ON(READ_ONCE(b->first_signal));
+	WARN_ON(rcu_access_pointer(b->first_signal));
 	WARN_ON(!RB_EMPTY_ROOT(&b->signals));
 
 	if (!IS_ERR_OR_NULL(b->signaler))
@@ -652,8 +660,8 @@ bool intel_breadcrumbs_busy(struct intel_engine_cs *engine)
 
 	spin_lock_irq(&b->lock);
 
-	if (b->first_wait) {
-		wake_up_process(b->first_wait->tsk);
+	if (rcu_access_pointer(b->first_signal)) {
+		wake_up_process(b->signaler);
 		busy |= intel_engine_flag(engine);
 	}
 
diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h
index 424b1eb38a8a..af50000fef3a 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.h
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
@@ -236,7 +236,7 @@ struct intel_engine_cs {
 		struct rb_root signals; /* sorted by retirement */
 		struct intel_wait *first_wait; /* oldest waiter by retirement */
 		struct task_struct *signaler; /* used for fence signalling */
-		struct drm_i915_gem_request *first_signal;
+		struct drm_i915_gem_request __rcu *first_signal;
 		struct timer_list fake_irq; /* used after a missed interrupt */
 		struct timer_list hangcheck; /* detect missed interrupts */
 
-- 
2.10.2