drm/i915: Eliminate the trylock for awaiting an earlier request

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@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191216165317.2742896-1-chris@chris-wilson.co.uk

diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
index af2f78e040d72271f150da73639ea934911f3db0..a59b803aef92c9a32e7beb5f2f6010e186c7592f 100644 (file)
--- a/drivers/gpu/drm/i915/i915_request.c
+++ b/drivers/gpu/drm/i915/i915_request.c
@@ -756,34 +756,37 @@ err_unlock:
 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))