[PATCH v2] drm/vmwgfx: Make sure unpinning handles reservations

[Thomas Hellström (Intel)]

Hi, Zack,

On 4/10/21 8:59 PM, Zack Rusin wrote:
> Quite often it's a little hard to tell if reservations are already held
> in code paths that unpin bo's. While our pinning/unpinning code should
> be more explicit that requires a substential amount of work so instead
> we can avoid the issues by making sure we try to reserve before unpinning.
> Because we unpin those bo's only on destruction/error paths just that check
> tells us if we're already reserved or not and allows to cleanly unpin.
>
> Reviewed-by: Martin Krastev <krastevm at vmware.com>
> Reviewed-by: Roland Scheidegger <sroland at vmware.com>
> Fixes: d1a73c641afd ("drm/vmwgfx: Make sure we unpin no longer needed buffers")
> Cc: dri-devel at lists.freedesktop.org
> Signed-off-by: Zack Rusin <zackr at vmware.com>
> ---
>   drivers/gpu/drm/vmwgfx/vmwgfx_drv.h | 11 +++++++++++
>   drivers/gpu/drm/vmwgfx/vmwgfx_mob.c | 18 ++++++++++++++----
>   2 files changed, 25 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_drv.h b/drivers/gpu/drm/vmwgfx/vmwgfx_drv.h
> index 8087a9013455..9a3133cb5cc1 100644
> --- a/drivers/gpu/drm/vmwgfx/vmwgfx_drv.h
> +++ b/drivers/gpu/drm/vmwgfx/vmwgfx_drv.h
> @@ -1523,8 +1523,19 @@ static inline void vmw_bo_unreference(struct vmw_buffer_object **buf)
>   
>   	*buf = NULL;
>   	if (tmp_buf != NULL) {
> +		/*
> +		 * currently pinning requires a reservation to be held
> +		 * but unreference can be called either reserved or not.
> +		 * To avoid deadloacks we have to try to get a reservation.
> +		 * This is exlusively to satisfy the requirements of the
> +		 * unpin, ttm_bo_put will handle delayed deletion for us
> +		 * in case the bo is really busy.
> +		 */
> +		bool locked = dma_resv_trylock(tmp_buf->base.base.resv);
>   		if (tmp_buf->base.pin_count > 0)
Hmm, Similar to the previous version, I figure someone another thread 
might be holding the lock and drop it here?
>   			ttm_bo_unpin(&tmp_buf->base);
And also, if this is a nested reference of a pinned buffer, you'd be 
incorrectly unpinning it.

/Thomas