[PATCH v2] drm/syncobj: Deal with signalled fences in drm_syncobj_find_fence.

[Christian König]

Am 08.12.21 um 03:39 schrieb Bas Nieuwenhuizen:
> dma_fence_chain_find_seqno only ever returns the top fence in the
> chain or an unsignalled fence. Hence if we request a seqno that
> is already signalled it returns a NULL fence. Some callers are
> not prepared to handle this, like the syncobj transfer functions
> for example.
>
> This behavior is "new" with timeline syncobj and it looks like
> not all callers were updated. To fix this behavior make sure
> that a successful drm_sync_find_fence always returns a non-NULL
> fence.
>
> v2: Move the fix to drm_syncobj_find_fence from the transfer
>      functions.
>
> Fixes: ea569910cbab ("drm/syncobj: add transition iotcls between binary and timeline v2")
> Cc: stable at vger.kernel.org
> Signed-off-by: Bas Nieuwenhuizen <bas at basnieuwenhuizen.nl>

Reviewed-by: Christian König <christian.koenig at amd.com>

> ---
>   drivers/gpu/drm/drm_syncobj.c | 11 ++++++++++-
>   1 file changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/drm_syncobj.c b/drivers/gpu/drm/drm_syncobj.c
> index fdd2ec87cdd1..11be91b5709b 100644
> --- a/drivers/gpu/drm/drm_syncobj.c
> +++ b/drivers/gpu/drm/drm_syncobj.c
> @@ -404,8 +404,17 @@ int drm_syncobj_find_fence(struct drm_file *file_private,
>   
>   	if (*fence) {
>   		ret = dma_fence_chain_find_seqno(fence, point);
> -		if (!ret)
> +		if (!ret) {
> +			/* If the requested seqno is already signaled
> +			 * drm_syncobj_find_fence may return a NULL
> +			 * fence. To make sure the recipient gets
> +			 * signalled, use a new fence instead.
> +			 */
> +			if (!*fence)
> +				*fence = dma_fence_get_stub();
> +
>   			goto out;
> +		}
>   		dma_fence_put(*fence);
>   	} else {
>   		ret = -EINVAL;