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

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Message-ID: <20190804184246.GD28441@linux.ibm.com>
Date:   Sun, 4 Aug 2019 11:42:46 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        mingo@...nel.org, jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com, joel@...lfernandes.org
Subject: Re: [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