[PATCH 11/24] jrcu: use xchg to toggle rcu_which

[Joe Korty <joe.korty@ccur.com>]

Use xchg rather than wmb()s to change the value
of rcu_which.

This may be better than wmb()s as xchg will not
let the insn counter advance past the xchg until
the exchanged result is visible to other cpus.

Signed-off-by: Joe Korty <joe.korty@ccur.com>

Index: b/kernel/jrcu.c
===================================================================
--- a/kernel/jrcu.c
+++ b/kernel/jrcu.c
@@ -371,14 +371,16 @@ static void __rcu_delimit_batches(struct
 		else
 			rd->wait = 0;
 	}
+	smp_wmb(); /* just paranoia, the below xchg should do this on all archs */
 
 	/*
-	 * Swap current and previous lists.  The other cpus must not see this
-	 * out-of-order w.r.t. the above emptying of each cpu's previous list,
-	 * hence the smp_wmb.
+	 * Swap current and previous lists.  The other cpus must not
+	 * see this out-of-order w.r.t. the above emptying of each cpu's
+	 * previous list.  The xchg accomplishes that and, as a side (but
+	 * seemingly unneeded) bonus, keeps this cpu from advancing its insn
+	 * counter until the results of that xchg are visible on other cpus.
 	 */
-	smp_wmb();
-	rcu_which = prev;
+	xchg(&rcu_which, prev); /* only place where rcu_which is written to */
 	rcu_stats.nbatches++;
 }