Re: [PATCH RFC tip/core/rcu 01/14] rcu/nocb: Atomic ->len field in rcu_segcblist structure

[Peter Zijlstra]

On Fri, Aug 02, 2019 at 08:14:48AM -0700, Paul E. McKenney wrote:
> +/*
> + * Exchange the numeric length of the specified rcu_segcblist structure
> + * with the specified value.  This can cause the ->len field to disagree
> + * with the actual number of callbacks on the structure.  This exchange is
> + * fully ordered with respect to the callers accesses both before and after.
> + */
> +long rcu_segcblist_xchg_len(struct rcu_segcblist *rsclp, long v)
> +{
> +#ifdef CONFIG_RCU_NOCB_CPU
> +	return atomic_long_xchg(&rsclp->len, v);
> +#else
> +	long ret = rsclp->len;
> +
> +	smp_mb(); /* Up to the caller! */
> +	WRITE_ONCE(rsclp->len, v);
> +	smp_mb(); /* Up to the caller! */
> +	return ret;
> +#endif
> +}

That one's weird; for matching semantics the load needs to be between
the memory barriers.