rcu: Simplify _rcu_barrier() processing

This commit drops an unneeded ACCESS_ONCE() and simplifies an "our work
is done" check in _rcu_barrier().  This applies feedback from Linus
(https://lkml.org/lkml/2013/7/26/777) that he gave to similar code
in an unrelated patch.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
[ paulmck: Fix comment to match code, reported by Lai Jiangshan. ]

1 files changed, 13 insertions, 2 deletions

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index c6a064abd6a0..a4a04f311cfb 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -2817,9 +2817,20 @@ static void _rcu_barrier(struct rcu_state *rsp)
          * transition.  The "if" expression below therefore rounds the old
          * value up to the next even number and adds two before comparing.
          */
-        snap_done = ACCESS_ONCE(rsp->n_barrier_done);
+        snap_done = rsp->n_barrier_done;
         _rcu_barrier_trace(rsp, "Check", -1, snap_done);
-        if (ULONG_CMP_GE(snap_done, ((snap + 1) & ~0x1) + 2)) {
+
+        /*
+         * If the value in snap is odd, we needed to wait for the current
+         * rcu_barrier() to complete, then wait for the next one, in other
+         * words, we need the value of snap_done to be three larger than
+         * the value of snap.  On the other hand, if the value in snap is
+         * even, we only had to wait for the next rcu_barrier() to complete,
+         * in other words, we need the value of snap_done to be only two
+         * greater than the value of snap.  The "(snap + 3) & ~0x1" computes
+         * this for us (thank you, Linus!).
+         */
+        if (ULONG_CMP_GE(snap_done, (snap + 3) & ~0x1)) {
                 _rcu_barrier_trace(rsp, "EarlyExit", -1, snap_done);
                 smp_mb(); /* caller's subsequent code after above check. */
                 mutex_unlock(&rsp->barrier_mutex);