Commit-ID: 458fb381eacdd23366cfa2fbdf5a467848683e3a
Gitweb: http://git.kernel.org/tip/458fb381eacdd23366cfa2fbdf5a467848683e3a
Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
AuthorDate: Fri, 26 Jul 2013 20:47:42 -0700
Committer: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
CommitDate: Tue, 20 Aug 2013 11:45:50 -0700
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@xxxxxxxxxxxxxxxxxx>
Reviewed-by: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
[ paulmck: Fix comment to match code, reported by Lai Jiangshan. ]
---
kernel/rcutree.c | 15 +++++++++++++--
1 file changed, 13 insertions(+), 2 deletions(-)
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index c6a064a..a4a04f3 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);
--
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