dma-buf: fix check in dma_resv_add_fence

It's valid to add the same fence multiple times to a dma-resv object and
we shouldn't need one extra slot for each.
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Fixes: a3f7c10a ("dma-buf/dma-resv: check if the new fence is really later")
Cc: stable@vger.kernel.org # v5.19+
Link: https://patchwork.freedesktop.org/patch/msgid/20231115093035.1889-1-christian.koenig@amd.com

drivers/dma-buf/dma-resv.c

@@ -301,7 +301,7 @@ void dma_resv_add_fence(struct dma_resv *obj, struct dma_fence *fence,
 
 		dma_resv_list_entry(fobj, i, obj, &old, &old_usage);
 		if ((old->context == fence->context && old_usage >= usage &&
-		     dma_fence_is_later(fence, old)) ||
+		     dma_fence_is_later_or_same(fence, old)) ||
 		    dma_fence_is_signaled(old)) {
 			dma_resv_list_set(fobj, i, fence, usage);
 			dma_fence_put(old);

include/linux/dma-fence.h

@@ -498,6 +498,21 @@ static inline bool dma_fence_is_later(struct dma_fence *f1,
 	return __dma_fence_is_later(f1->seqno, f2->seqno, f1->ops);
 }
 
+/**
+ * dma_fence_is_later_or_same - return true if f1 is later or same as f2
+ * @f1: the first fence from the same context
+ * @f2: the second fence from the same context
+ *
+ * Returns true if f1 is chronologically later than f2 or the same fence. Both
+ * fences must be from the same context, since a seqno is not re-used across
+ * contexts.
+ */
+static inline bool dma_fence_is_later_or_same(struct dma_fence *f1,
+					      struct dma_fence *f2)
+{
+	return f1 == f2 || dma_fence_is_later(f1, f2);
+}
+
 /**
  * dma_fence_later - return the chronologically later fence
  * @f1:	the first fence from the same context