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

This is to match the concurrent version, which uses atomic_xchg().

Thanx, Paul

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