From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751992Ab3HTCm1 (ORCPT ); Mon, 19 Aug 2013 22:42:27 -0400 Received: from e33.co.us.ibm.com ([32.97.110.151]:39638 "EHLO e33.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751810Ab3HTCmZ (ORCPT ); Mon, 19 Aug 2013 22:42:25 -0400 From: "Paul E. McKenney" To: linux-kernel@vger.kernel.org Cc: mingo@elte.hu, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@polymtl.ca, josh@joshtriplett.org, niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, darren@dvhart.com, fweisbec@gmail.com, sbw@mit.edu, "Paul E. McKenney" Subject: [PATCH tip/core/rcu 8/9] rcu: Simplify _rcu_barrier() processing Date: Mon, 19 Aug 2013 19:42:13 -0700 Message-Id: <1376966534-30775-8-git-send-email-paulmck@linux.vnet.ibm.com> X-Mailer: git-send-email 1.8.1.5 In-Reply-To: <1376966534-30775-1-git-send-email-paulmck@linux.vnet.ibm.com> References: <20130820024148.GA30283@linux.vnet.ibm.com> <1376966534-30775-1-git-send-email-paulmck@linux.vnet.ibm.com> X-TM-AS-MML: No X-Content-Scanned: Fidelis XPS MAILER x-cbid: 13082002-2398-0000-0000-000001A46F2A Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: "Paul E. McKenney" 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 Reviewed-by: Josh Triplett --- kernel/rcutree.c | 15 +++++++++++++-- 1 file changed, 13 insertions(+), 2 deletions(-) diff --git a/kernel/rcutree.c b/kernel/rcutree.c index c6a064a..612aff1 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); -- 1.8.1.5