drm/i915/gt: Protect execlists_hold/unhold from new waiters

As we may add new waiters to a request as it is being run, we need to
mark the list iteration as being safe for concurrent addition.

v2: Mika spotted that we used the same trick for signalers_list, so warn
the compiler about the lockless walk there as well.

Fixes: 32ff621f ("drm/i915/gt: Allow temporary suspension of inflight requests")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200207110213.2734386-1-chris@chris-wilson.co.uk

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

@@ -1620,6 +1620,11 @@ last_active(const struct intel_engine_execlists *execlists)
 				     &(rq__)->sched.waiters_list, \
 				     wait_link)
 
+#define for_each_signaler(p__, rq__) \
+	list_for_each_entry_lockless(p__, \
+				     &(rq__)->sched.signalers_list, \
+				     signal_link)
+
 static void defer_request(struct i915_request *rq, struct list_head * const pl)
 {
 	LIST_HEAD(list);
@@ -2378,7 +2383,7 @@ static void __execlists_hold(struct i915_request *rq)
 		list_move_tail(&rq->sched.link, &rq->engine->active.hold);
 		i915_request_set_hold(rq);
 
-		list_for_each_entry(p, &rq->sched.waiters_list, wait_link) {
+		for_each_waiter(p, rq) {
 			struct i915_request *w =
 				container_of(p->waiter, typeof(*w), sched);
 
@@ -2464,7 +2469,7 @@ static bool hold_request(const struct i915_request *rq)
 	 * If one of our ancestors is on hold, we must also be on hold,
 	 * otherwise we will bypass it and execute before it.
 	 */
-	list_for_each_entry(p, &rq->sched.signalers_list, signal_link) {
+	for_each_signaler(p, rq) {
 		const struct i915_request *s =
 			container_of(p->signaler, typeof(*s), sched);
 
@@ -2496,7 +2501,7 @@ static void __execlists_unhold(struct i915_request *rq)
 		RQ_TRACE(rq, "hold release\n");
 
 		/* Also release any children on this engine that are ready */
-		list_for_each_entry(p, &rq->sched.waiters_list, wait_link) {
+		for_each_waiter(p, rq) {
 			struct i915_request *w =
 				container_of(p->waiter, typeof(*w), sched);
 

drivers/gpu/drm/i915/i915_scheduler.c

@@ -433,7 +433,7 @@ bool __i915_sched_node_add_dependency(struct i915_sched_node *node,
 			node->flags |= I915_SCHED_HAS_SEMAPHORE_CHAIN;
 
 		/* All set, now publish. Beware the lockless walkers. */
-		list_add(&dep->signal_link, &node->signalers_list);
+		list_add_rcu(&dep->signal_link, &node->signalers_list);
 		list_add_rcu(&dep->wait_link, &signal->waiters_list);
 
 		/*