Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Sun, 4 Aug 2019 16:50:51 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Paul E. McKenney" <paulmck@linux.ibm.com>
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
Message-ID: <20190804145051.GG2349@hirez.programming.kicks-ass.net>
References: <20190802151435.GA1081@linux.ibm.com>
 <20190802151501.13069-1-paulmck@linux.ibm.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20190802151501.13069-1-paulmck@linux.ibm.com>
User-Agent: Mutt/1.10.1 (2018-07-13)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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.