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@xxxxxxxxxxxxxxxxxx>
-Chris
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