dma-buf: drop the DAG approach for the dma_resv object v3

So far we had the approach of using a directed acyclic
graph with the dma_resv obj.

This turned out to have many downsides, especially it means
that every single driver and user of this interface needs
to be aware of this restriction when adding fences. If the
rules for the DAG are not followed then we end up with
potential hard to debug memory corruption, information
leaks or even elephant big security holes because we allow
userspace to access freed up memory.

Since we already took a step back from that by always
looking at all fences we now go a step further and stop
dropping the shared fences when a new exclusive one is
added.

v2: Drop some now superflous documentation
v3: Add some more documentation for the new handling.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20220321135856.1331-11-christian.koenig@amd.com

1 files changed, 1 insertions, 15 deletions

diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c
index 5001e9b4420a..be65522f0f47 100644
--- a/drivers/dma-buf/dma-resv.c
+++ b/drivers/dma-buf/dma-resv.c
@@ -351,35 +351,21 @@ EXPORT_SYMBOL(dma_resv_replace_fences);
  * @fence: the exclusive fence to add
  *
  * Add a fence to the exclusive slot. @obj must be locked with dma_resv_lock().
- * Note that this function replaces all fences attached to @obj, see also
- * &dma_resv.fence_excl for a discussion of the semantics.
+ * See also &dma_resv.fence_excl for a discussion of the semantics.
  */
 void dma_resv_add_excl_fence(struct dma_resv *obj, struct dma_fence *fence)
 {
 	struct dma_fence *old_fence = dma_resv_excl_fence(obj);
-	struct dma_resv_list *old;
-	u32 i = 0;
 
 	dma_resv_assert_held(obj);
 
-	old = dma_resv_shared_list(obj);
-	if (old)
-		i = old->shared_count;
-
 	dma_fence_get(fence);
 
 	write_seqcount_begin(&obj->seq);
 	/* write_seqcount_begin provides the necessary memory barrier */
 	RCU_INIT_POINTER(obj->fence_excl, fence);
-	if (old)
-		old->shared_count = 0;
 	write_seqcount_end(&obj->seq);
 
-	/* inplace update, no shared fences */
-	while (i--)
-		dma_fence_put(rcu_dereference_protected(old->shared[i],
-						dma_resv_held(obj)));
-
 	dma_fence_put(old_fence);
 }
 EXPORT_SYMBOL(dma_resv_add_excl_fence);