[Intel-gfx] [PATCH 10/10] drm/i915: Drop i915_request.lock serialisation around await_start

[Chris Wilson]

Originally, we used the signal->lock as a means of following the
previous link in its timeline and peeking at the previous fence.
However, we have replaced the explicit serialisation with a series of
very careful probes that anticipate the links being deleted and the
fences recycled before we are able to acquire a strong reference to it.
We do not need the signal->lock crutch anymore, nor want the contention.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
---
 drivers/gpu/drm/i915/i915_request.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
index 4ebb0f144ac4..9f369c33ca49 100644
--- a/drivers/gpu/drm/i915/i915_request.c
+++ b/drivers/gpu/drm/i915/i915_request.c
@@ -955,9 +955,16 @@ i915_request_await_start(struct i915_request *rq, struct i915_request *signal)
 	if (i915_request_started(signal))
 		return 0;
 
+	/*
+	 * The caller holds a reference on @signal, but we do not serialise
+	 * against it being retired and removed from the lists.
+	 *
+	 * We do not hold a reference to the request before @signal, and
+	 * so must be very careful to ensure that it is not _recycled_ as
+	 * we follow the link backwards.
+	 */
 	fence = NULL;
 	rcu_read_lock();
-	spin_lock_irq(&signal->lock);
 	do {
 		struct list_head *pos = READ_ONCE(signal->link.prev);
 		struct i915_request *prev;
@@ -988,7 +995,6 @@ i915_request_await_start(struct i915_request *rq, struct i915_request *signal)
 
 		fence = &prev->fence;
 	} while (0);
-	spin_unlock_irq(&signal->lock);
 	rcu_read_unlock();
 	if (!fence)
 		return 0;
-- 
2.20.1