[Intel-gfx] [PATCH] drm/i915/gt: Protect signaler walk with RCU

[Chris Wilson]

While we know that the waiters cannot disappear as we walk our list
(only that they might be added), the same cannot be said for our
signalers as they may be completed by the HW and retired as we process
this request. Ergo we need to use rcu to protect the list iteration and
remember to mark up the list_del_rcu.

Fixes: 793c22617367 ("drm/i915/gt: Protect execlists_hold/unhold from new waiters")
Fixes: 32ff621fd744 ("drm/i915/gt: Allow temporary suspension of inflight requests")
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
Cc: Mika Kuoppala <mika.kuoppala at linux.intel.com>
---
 drivers/gpu/drm/i915/gt/intel_lrc.c   | 16 ++++++++++------
 drivers/gpu/drm/i915/i915_scheduler.c |  4 ++--
 2 files changed, 12 insertions(+), 8 deletions(-)

diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
index ba31cbe8c68e..47561dc29304 100644
--- a/drivers/gpu/drm/i915/gt/intel_lrc.c
+++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
@@ -1668,9 +1668,9 @@ last_active(const struct intel_engine_execlists *execlists)
 				     wait_link)
 
 #define for_each_signaler(p__, rq__) \
-	list_for_each_entry_lockless(p__, \
-				     &(rq__)->sched.signalers_list, \
-				     signal_link)
+	list_for_each_entry_rcu(p__, \
+				&(rq__)->sched.signalers_list, \
+				signal_link)
 
 static void defer_request(struct i915_request *rq, struct list_head * const pl)
 {
@@ -2533,11 +2533,13 @@ static bool execlists_hold(struct intel_engine_cs *engine,
 static bool hold_request(const struct i915_request *rq)
 {
 	struct i915_dependency *p;
+	bool result = false;
 
 	/*
 	 * If one of our ancestors is on hold, we must also be on hold,
 	 * otherwise we will bypass it and execute before it.
 	 */
+	rcu_read_lock();
 	for_each_signaler(p, rq) {
 		const struct i915_request *s =
 			container_of(p->signaler, typeof(*s), sched);
@@ -2545,11 +2547,13 @@ static bool hold_request(const struct i915_request *rq)
 		if (s->engine != rq->engine)
 			continue;
 
-		if (i915_request_on_hold(s))
-			return true;
+		result = i915_request_on_hold(s);
+		if (result)
+			break;
 	}
+	rcu_read_unlock();
 
-	return false;
+	return result;
 }
 
 static void __execlists_unhold(struct i915_request *rq)
diff --git a/drivers/gpu/drm/i915/i915_scheduler.c b/drivers/gpu/drm/i915/i915_scheduler.c
index e19a37a83397..4e48757e9de6 100644
--- a/drivers/gpu/drm/i915/i915_scheduler.c
+++ b/drivers/gpu/drm/i915/i915_scheduler.c
@@ -486,7 +486,7 @@ void i915_sched_node_fini(struct i915_sched_node *node)
 	list_for_each_entry_safe(dep, tmp, &node->signalers_list, signal_link) {
 		GEM_BUG_ON(!list_empty(&dep->dfs_link));
 
-		list_del(&dep->wait_link);
+		list_del_rcu(&dep->wait_link);
 		if (dep->flags & I915_DEPENDENCY_ALLOC)
 			i915_dependency_free(dep);
 	}
@@ -497,7 +497,7 @@ void i915_sched_node_fini(struct i915_sched_node *node)
 		GEM_BUG_ON(dep->signaler != node);
 		GEM_BUG_ON(!list_empty(&dep->dfs_link));
 
-		list_del(&dep->signal_link);
+		list_del_rcu(&dep->signal_link);
 		if (dep->flags & I915_DEPENDENCY_ALLOC)
 			i915_dependency_free(dep);
 	}
-- 
2.25.0