Commit b2fe00bb authored by Chris Wilson's avatar Chris Wilson
Browse files

drm/i915: Drop i915_request.lock serialisation around await_start 



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: default avatarChris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: default avatarAndi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210114135612.13210-2-chris@chris-wilson.co.uk
parent 163433e5

drivers/gpu/drm/i915/i915_request.c

+8 −2
Original line number Diff line number Diff line
@@ -969,9 +969,16 @@ 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;
@@ -1002,7 +1009,6 @@ 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;