dma-fence: Store the timestamp in the same union as the cb_list

The timestamp and the cb_list are mutually exclusive, the cb_list can
only be added to prior to being signaled (and once signaled we drain),
while the timestamp is only valid upon being signaled. Both the
timestamp and the cb_list are only valid while the fence is alive, and
as soon as no references are held can be replaced by the rcu_head.

By reusing the union for the timestamp, we squeeze the base dma_fence
struct to 64 bytes on x86-64.

v2: Sort the union chronologically

Suggested-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Christian König <christian.koenig@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>.
Link: https://patchwork.freedesktop.org/patch/msgid/20190817153022.5749-1-chris@chris-wilson.co.uk

1 files changed, 9 insertions, 7 deletions

diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
index 8a6d0250285d..2c136aee3e79 100644
--- a/drivers/dma-buf/dma-fence.c
+++ b/drivers/dma-buf/dma-fence.c
@@ -129,6 +129,7 @@ EXPORT_SYMBOL(dma_fence_context_alloc);
 int dma_fence_signal_locked(struct dma_fence *fence)
 {
         struct dma_fence_cb *cur, *tmp;
+        struct list_head cb_list;
 
         lockdep_assert_held(fence->lock);
 
@@ -136,16 +137,16 @@ int dma_fence_signal_locked(struct dma_fence *fence)
                                       &fence->flags)))
                 return -EINVAL;
 
+        /* Stash the cb_list before replacing it with the timestamp */
+        list_replace(&fence->cb_list, &cb_list);
+
         fence->timestamp = ktime_get();
         set_bit(DMA_FENCE_FLAG_TIMESTAMP_BIT, &fence->flags);
         trace_dma_fence_signaled(fence);
 
-        if (!list_empty(&fence->cb_list)) {
-                list_for_each_entry_safe(cur, tmp, &fence->cb_list, node) {
-                        INIT_LIST_HEAD(&cur->node);
-                        cur->func(fence, cur);
-                }
-                INIT_LIST_HEAD(&fence->cb_list);
+        list_for_each_entry_safe(cur, tmp, &cb_list, node) {
+                INIT_LIST_HEAD(&cur->node);
+                cur->func(fence, cur);
         }
 
         return 0;
@@ -231,7 +232,8 @@ void dma_fence_release(struct kref *kref)
 
         trace_dma_fence_destroy(fence);
 
-        if (WARN(!list_empty(&fence->cb_list),
+        if (WARN(!list_empty(&fence->cb_list) &&
+                 !test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags),
                  "Fence %s:%s:%llx:%llx released with pending signals!\n",
                  fence->ops->get_driver_name(fence),
                  fence->ops->get_timeline_name(fence),