rcu classic: simplify the next pending batch

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>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Dipankar Sarma <dipankar@in.ibm.com>
Cc: Gautham Shenoy <ego@in.ibm.com>
Cc: Dhaval Giani <dhaval@linux.vnet.ibm.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

2 files changed, 9 insertions, 15 deletions

diff --git a/include/linux/rcuclassic.h b/include/linux/rcuclassic.h
index 8c774905dcfe..c847e59c6006 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 16eeeaa9d618..03726eb95193 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++;
 
                 /*
@@ -441,16 +437,14 @@ 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);
                 }
         }