linux-kernel - [PATCH 8/8] srcu: use ACCESS_ONCE() to access sp->completed in srcu_read

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Message-Id: <1354178770-6028-9-git-send-email-laijs@cn.fujitsu.com>
Date:	Thu, 29 Nov 2012 16:46:09 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	linux-kernel@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Lai Jiangshan <laijs@...fujitsu.com>
Subject: [PATCH 8/8] srcu: use ACCESS_ONCE() to access sp->completed in srcu_read_lock()

Old srcu implement requires sp->completed is loaded in
RCU-sched(preempt_disable()) section.

The new srcu is now not RCU-sched based, it doesn't require the load of
sp->completed and the access to counter must be in the same RCU-sched
read site C.S., so we use ACCESS_ONCE() instead, and move it out of
the preempt_disable() section, preempt_disable() section is only used
for percpu data, not for RCU-sched.

The resulted code is almost as the same as before, but it helps people to
understand the code, and it avoids to add surprise to reviewer: "why we need
rcu_read_lock_sched_held() here?"

Signed-off-by: Lai Jiangshan <laijs@...fujitsu.com>
---
 kernel/srcu.c |    3 +--
 1 files changed, 1 insertions(+), 2 deletions(-)

diff --git a/kernel/srcu.c b/kernel/srcu.c
index 38a762f..224400a 100644
--- a/kernel/srcu.c
+++ b/kernel/srcu.c
@@ -294,9 +294,8 @@ int __srcu_read_lock(struct srcu_struct *sp)
 {
 	int idx;
 
+	idx = ACCESS_ONCE(sp->completed) & 0x1;
 	preempt_disable();
-	idx = rcu_dereference_index_check(sp->completed,
-					  rcu_read_lock_sched_held()) & 0x1;
 	ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->c[idx]) += 1;
 	smp_mb(); /* B */  /* Avoid leaking the critical section. */
 	ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->seq[idx]) += 1;
-- 
1.7.4.4

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