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>

diff --git a/include/linux/rcuclassic.h b/include/linux/rcuclassic.h
index 8c77490..c847e59 100644 (file)
--- 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 16eeeaa..03726eb 100644 (file)
--- 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);
                }
        }