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

[Chris Wilson <chris@chris-wilson.co.uk>]

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