Re: [Intel-gfx] [PATCH] drm/i915: Use dma_resv_iter for waiting in i915_gem_object_wait_reservation.

[Tvrtko Ursulin]

On 13/10/2021 11:41, Maarten Lankhorst wrote:
No memory should be allocated when calling i915_gem_object_wait,
because it may be called to idle a BO when evicting memory.

Fix this by using dma_resv_iter helpers to call
i915_gem_object_wait_fence() on each fence, which cleans up the code a lot.
Also remove dma_resv_prune, it's questionably.

This will result in the following lockdep splat.

<snip>

@@ -37,56 +36,17 @@ i915_gem_object_wait_reservation(struct dma_resv *resv,
                                 unsigned int flags,
                                 long timeout)
  {
-       struct dma_fence *excl;
-       bool prune_fences = false;
-
-       if (flags & I915_WAIT_ALL) {
-               struct dma_fence **shared;
-               unsigned int count, i;
-               int ret;
+       struct dma_resv_iter cursor;
+       struct dma_fence *fence;
  
-		ret = dma_resv_get_fences(resv, &excl, &count, &shared);
-               if (ret)
-                       return ret;
-
-               for (i = 0; i < count; i++) {
-                       timeout = i915_gem_object_wait_fence(shared[i],
-                                                            flags, timeout);
-                       if (timeout < 0)
-                               break;
+       dma_resv_iter_begin(&cursor, resv, flags & I915_WAIT_ALL);
+       dma_resv_for_each_fence_unlocked(&cursor, fence) {
  
-			dma_fence_put(shared[i]);
-               }
-
-               for (; i < count; i++)
-                       dma_fence_put(shared[i]);
-               kfree(shared);
-
-               /*
-                * If both shared fences and an exclusive fence exist,
-                * then by construction the shared fences must be later
-                * than the exclusive fence. If we successfully wait for
-                * all the shared fences, we know that the exclusive fence
-                * must all be signaled. If all the shared fences are
-                * signaled, we can prune the array and recover the
-                * floating references on the fences/requests.
-                */
-               prune_fences = count && timeout >= 0;
-       } else {
-               excl = dma_resv_get_excl_unlocked(resv);
+               timeout = i915_gem_object_wait_fence(fence, flags, timeout);
+               if (timeout <= 0)
+                       break;

You have another change in behaviour here, well a bug really. When 
userspace passes in zero timeout you fail to report activity in other 
than the first fence.

Regards,

Tvrtko