[PATCH 7/8] dma-buf: add reservation_object_fences helper

[Chris Wilson]

Quoting Christian König (2019-08-06 16:01:33)
> Add a new helper to get a consistent set of pointers from the reservation
> object. While at it group all access helpers together in the header file.

Ah, needs to be earlier :)
 
> +/**
> + * reservation_object_fences - read consistent fence pointers
> + * @obj: reservation object where we get the fences from
> + * @excl: pointer for the exclusive fence
> + * @list: pointer for the shared fence list
> + *
> + * Make sure we have a consisten exclusive fence and shared fence list.
> + * Must be called with rcu read side lock held.
> + */
> +static inline void
> +reservation_object_fences(struct reservation_object *obj,
> +                         struct dma_fence **excl,
> +                         struct reservation_object_list **list)
> +{
> +       unsigned int seq;
> +
> +       do {
> +               seq = read_seqcount_begin(&obj->seq);
> +               *excl = rcu_dereference(obj->fence_excl);
> +               *list = rcu_dereference(obj->fence);
> +       } while (read_seqcount_retry(&obj->seq, seq));
> +}

I would personally prefer return excl rather than have it as a second
outparam, but I'd leave that to gcc to decide.

Having stared at this, I agree this does the right thing. The important
point from all callers' perspective is that the combination of pointers
is consistent for this rcu_read_lock. And rcu_dereference enforces the
callers do hold rcu_read_lock.

I didn't check all the conversions, just stared at the heart of the
problem.

Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>
-Chris