Quoting Michel Dänzer (2018-06-26 15:31:47)
From: Michel Dänzer michel.daenzer@amd.com
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@vger.kernel.org Signed-off-by: Michel Dänzer michel.daenzer@amd.com
I've convinced myself (or rather have not found a valid argument against) that being able to call reserve_shared + add_shared multiple times for the same fence is an intended part of reservation_object API
I'd double check with Christian though.
Reviewed-by: Chris Wilson chris@chris-wilson.co.uk
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);
Personally I would just let kasan detect this and throw away the BUG_ON or at least move it behind some DMABUF_BUG_ON(). -Chris