[Intel-gfx] [PATCH v3] drm/i915: Stop listening to request resubmission from the signaler kthread

[Chris Wilson]

The intent here was that we would be listening to
i915_gem_request_unsubmit in order to cancel the signaler quickly and
release the reference on the request. Cancelling the signaler is done
directly via intel_engine_cancel_signaling (called from unsubmit), but
that does not directly wake up the signaling thread, and neither does
setting the request->global_seqno back to zero wake up listeners to the
request->execute waitqueue. So the only time that listening to the
request->execute waitqueue would wake up the signaling kthread would be
on the request resubmission, during which time we would already receive
wake ups from rejoining the global breadcrumbs wait rbtree.

Trying to wake up to release the request remains an issue. If the
signaling was cancelled and no other request required signaling, then it
is possible for us to shutdown with the reference on the request still
held. To ensure that we do not try to shutdown, leaking that request, we
must release that reference on kthread_should_stop().

v2: Defer releasing the local reference until the very end of the
thread, if we happen to never reuse the kthread. We should then still
free the request before we finalize the kmem caches.

Fixes: d6a2289d9d6b ("drm/i915: Remove the preempted request from the execution queue")
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin at linux.intel.com>
Cc: Michał Winiarski <michal.winiarski at intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin at linux.intel.com> #v1
---
 drivers/gpu/drm/i915/intel_breadcrumbs.c | 12 ++----------
 1 file changed, 2 insertions(+), 10 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_breadcrumbs.c b/drivers/gpu/drm/i915/intel_breadcrumbs.c
index 24c6fefdd0b1..0c3b321cac3e 100644
--- a/drivers/gpu/drm/i915/intel_breadcrumbs.c
+++ b/drivers/gpu/drm/i915/intel_breadcrumbs.c
@@ -686,23 +686,15 @@ static int intel_breadcrumbs_signaler(void *arg)
 		}
 
 		if (unlikely(do_schedule)) {
-			DEFINE_WAIT(exec);
-
 			if (kthread_should_park())
 				kthread_parkme();
 
-			if (kthread_should_stop()) {
-				GEM_BUG_ON(request);
+			if (unlikely(kthread_should_stop())) {
+				i915_gem_request_put(request);
 				break;
 			}
 
-			if (request)
-				add_wait_queue(&request->execute, &exec);
-
 			schedule();
-
-			if (request)
-				remove_wait_queue(&request->execute, &exec);
 		}
 		i915_gem_request_put(request);
 	} while (1);
-- 
2.15.1