[PATCH] dma-buf: Move BUG_ON from _add_shared_fence to _add_shared_inplace

[Michel DÃnzer]

From: Michel DÃnzer <michel.daenzer@xxxxxxx>

Fixes the BUG_ON spuriously triggering under the following
circumstances:

* ttm_eu_reserve_buffers processes a list containing multiple BOs using
  the same reservation object, so it calls
  reservation_object_reserve_shared with that reservation object once
  for each such BO.
* In reservation_object_reserve_shared, old->shared_count ==
  old->shared_max - 1, so obj->staged is freed in preparation of an
  in-place update.
* ttm_eu_fence_buffer_objects calls reservation_object_add_shared_fence
  once for each of the BOs above, always with the same fence.
* The first call adds the fence in the remaining free slot, after which
  old->shared_count == old->shared_max.

In the next call to reservation_object_add_shared_fence, the BUG_ON
triggers. However, nothing bad would happen in
reservation_object_add_shared_inplace, since the fence is already in the
reservation object.

Prevent this by moving the BUG_ON to where an overflow would actually
happen (e.g. if a buggy caller didn't call
reservation_object_reserve_shared before).

Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Michel DÃnzer <michel.daenzer@xxxxxxx>
---
 drivers/dma-buf/reservation.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/dma-buf/reservation.c b/drivers/dma-buf/reservation.c
index 314eb1071cce..532545b9488e 100644
--- a/drivers/dma-buf/reservation.c
+++ b/drivers/dma-buf/reservation.c
@@ -141,6 +141,7 @@ reservation_object_add_shared_inplace(struct reservation_object *obj,
 	if (signaled) {
 		RCU_INIT_POINTER(fobj->shared[signaled_idx], fence);
 	} else {
+		BUG_ON(fobj->shared_count >= fobj->shared_max);
 		RCU_INIT_POINTER(fobj->shared[fobj->shared_count], fence);
 		fobj->shared_count++;
 	}
@@ -230,10 +231,9 @@ void reservation_object_add_shared_fence(struct reservation_object *obj,
 	old = reservation_object_get_list(obj);
 	obj->staged = NULL;
 
-	if (!fobj) {
-		BUG_ON(old->shared_count >= old->shared_max);
+	if (!fobj)
 		reservation_object_add_shared_inplace(obj, old, fence);
-	} else
+	else
 		reservation_object_add_shared_replace(obj, old, fobj, fence);
 }
 EXPORT_SYMBOL(reservation_object_add_shared_fence);
-- 
2.18.0