Am 26.06.2018 um 16:31 schrieb Michel Dänzer:
> From: Michel Dänzer <michel.daenzer at 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.
Well, the explanation here is not correct. For multiple BOs using the
same reservation object we won't call
reservation_object_add_shared_fence() multiple times because we move
those to the duplicates list in ttm_eu_reserve_buffers().
But this bug can still happen because we call
reservation_object_add_shared_fence() manually with fences for the same
context in a couple of places.
One prominent case which comes to my mind are for the VM BOs during
updates. Another possibility are VRAM BOs which need to be cleared.
>
> 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 at vger.kernel.org
> Signed-off-by: Michel Dänzer <michel.daenzer at amd.com>
Reviewed-by: Christian König <christian.koenig at amd.com>.
Regards,
Christian.
> ---
> 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);
