We currently use an error-prone mutex_trylock to grab another timeline
to find an earlier request along it. However, with a bit of a
sleight-of-hand, we can reduce the mutex_trylock to a spin_lock on the
immediate request and careful pointer chasing to acquire a reference on
the previous request.
Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
---
drivers/gpu/drm/i915/i915_request.c | 39 ++++++++++++++++-------------
1 file changed, 21 insertions(+), 18 deletions(-)
diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
index fb8738987aeb..f513747ff027 100644
--- a/drivers/gpu/drm/i915/i915_request.c
+++ b/drivers/gpu/drm/i915/i915_request.c
@@ -768,34 +768,37 @@ i915_request_create(struct intel_context *ce)
static int
i915_request_await_start(struct i915_request *rq, struct i915_request *signal)
{
- struct intel_timeline *tl;
struct dma_fence *fence;
int err;
GEM_BUG_ON(i915_request_timeline(rq) ==
rcu_access_pointer(signal->timeline));
+ fence = NULL;
rcu_read_lock();
- tl = rcu_dereference(signal->timeline);
- if (i915_request_started(signal) || !kref_get_unless_zero(&tl->kref))
- tl = NULL;
- rcu_read_unlock();
- if (!tl) /* already started or maybe even completed */
- return 0;
+ spin_lock_irq(&signal->lock);
+ if (!i915_request_started(signal) &&
+ !list_is_first(&signal->link,
+ &rcu_dereference(signal->timeline)->requests)) {
+ struct i915_request *prev = list_prev_entry(signal, link);
- fence = ERR_PTR(-EAGAIN);
- if (mutex_trylock(&tl->mutex)) {
- fence = NULL;
- if (!i915_request_started(signal) &&
- !list_is_first(&signal->link, &tl->requests)) {
- signal = list_prev_entry(signal, link);
- fence = dma_fence_get(&signal->fence);
+ /*
+ * Peek at the request before us in the timeline. That
+ * request will only be valid before it is retired, so
+ * after acquiring a reference to it, confirm that it is
+ * still part of the signaler's timeline.
+ */
+ if (i915_request_get_rcu(prev)) {
+ if (list_next_entry(prev, link) == signal)
+ fence = &prev->fence;
+ else
+ i915_request_put(prev);
}
- mutex_unlock(&tl->mutex);
}
- intel_timeline_put(tl);
- if (IS_ERR_OR_NULL(fence))
- return PTR_ERR_OR_ZERO(fence);
+ spin_unlock_irq(&signal->lock);
+ rcu_read_unlock();
+ if (!fence)
+ return 0;
err = 0;
if (intel_timeline_sync_is_later(i915_request_timeline(rq), fence))
--
2.24.0
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