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Date:   Sun, 4 Aug 2019 11:45:23 -0700
From:   "Paul E. McKenney" <paulmck@linux.ibm.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     rcu@vger.kernel.org, linux-kernel@vger.kernel.org,
        mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com,
        akpm@linux-foundation.org, mathieu.desnoyers@efficios.com,
        josh@joshtriplett.org, tglx@linutronix.de, rostedt@goodmis.org,
        dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com,
        oleg@redhat.com, joel@joelfernandes.org
Subject: Re: [PATCH RFC tip/core/rcu 01/14] rcu/nocb: Atomic ->len field in
 rcu_segcblist structure
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On Sun, Aug 04, 2019 at 04:52:46PM +0200, Peter Zijlstra wrote:
> 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().

Not in this case, because ->len is written only by the CPU in question
in the !RCU_NOCB_CPU case.

It would not be hard to convince me that adding READ_ONCE() would be
cheap and easy future-proofing, but Linus has objected to that sort of
thing in the past.

							Thanx, Paul