[RFC][PATCH 1/2] rcu classic: simplify the next pending batch

[Lai Jiangshan <laijs@cn.fujitsu.com>]

use a batch number(rcp->pending) instead of a flag(rcp->next_pending)

rcu_start_batch() need to change this flag, so mb()s is needed
for memory-access safe.

but(after this patch applied) rcu_start_batch() do not change
this batch number(rcp->pending), rcp->pending is managed by
__rcu_process_callbacks only, and troublesome mb()s are eliminated.

And codes look simpler and clearer.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
---
diff --git a/include/linux/rcuclassic.h b/include/linux/rcuclassic.h
index b3aa05b..5efd734 100644
--- a/include/linux/rcuclassic.h
+++ b/include/linux/rcuclassic.h
@@ -45,7 +45,7 @@
 struct rcu_ctrlblk {
 	long	cur;		/* Current batch number.                      */
 	long	completed;	/* Number of the last completed batch         */
-	int	next_pending;	/* Is the next batch already waiting?         */
+	long	pending;	/* Number of the last pending batch           */
 
 	int	signaled;
 
diff --git a/kernel/rcuclassic.c b/kernel/rcuclassic.c
index a38895a..9ac3b94 100644
--- a/kernel/rcuclassic.c
+++ b/kernel/rcuclassic.c
@@ -60,12 +60,14 @@ EXPORT_SYMBOL_GPL(rcu_lock_map);
 static struct rcu_ctrlblk rcu_ctrlblk = {
 	.cur = -300,
 	.completed = -300,
+	.pending = -300,
 	.lock = __SPIN_LOCK_UNLOCKED(&rcu_ctrlblk.lock),
 	.cpumask = CPU_MASK_NONE,
 };
 static struct rcu_ctrlblk rcu_bh_ctrlblk = {
 	.cur = -300,
 	.completed = -300,
+	.pending = -300,
 	.lock = __SPIN_LOCK_UNLOCKED(&rcu_bh_ctrlblk.lock),
 	.cpumask = CPU_MASK_NONE,
 };
@@ -276,14 +278,8 @@ static void rcu_do_batch(struct rcu_data *rdp)
  */
 static void rcu_start_batch(struct rcu_ctrlblk *rcp)
 {
-	if (rcp->next_pending &&
+	if (rcp->cur != rcp->pending &&
 			rcp->completed == rcp->cur) {
-		rcp->next_pending = 0;
-		/*
-		 * next_pending == 0 must be visible in
-		 * __rcu_process_callbacks() before it can see new value of cur.
-		 */
-		smp_wmb();
 		rcp->cur++;
 
 		/*
@@ -437,16 +433,14 @@ static void __rcu_process_callbacks(struct rcu_ctrlblk *rcp,
 
 		/* determine batch number */
 		rdp->batch = rcp->cur + 1;
-		/* see the comment and corresponding wmb() in
-		 * the rcu_start_batch()
-		 */
-		smp_rmb();
 
-		if (!rcp->next_pending) {
+		if (rcu_batch_after(rdp->batch, rcp->pending)) {
 			/* and start it/schedule start if it's a new batch */
 			spin_lock(&rcp->lock);
-			rcp->next_pending = 1;
-			rcu_start_batch(rcp);
+			if (rcu_batch_after(rdp->batch, rcp->pending)) {
+				rcp->pending = rdp->batch;
+				rcu_start_batch(rcp);
+			}
 			spin_unlock(&rcp->lock);
 		}
 	}