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

[Peter Zijlstra]

On Sun, Aug 04, 2019 at 04:50:51PM +0200, Peter Zijlstra wrote:
> 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.

Also, since you WRITE_ONCE() the thing, the load needs to be a
READ_ONCE().