[patch] Avoid use of spinlock for percpu_counter

[Ravikiran G Thirumalai <kiran@scalex86.org>]

The spinlock in struct percpu_counter protects just one counter.  It's
not obvious why it was done this way (I am guessing it was because earlier,
atomic_t was guaranteed 24 bits only on some arches).  Since we have
atomic_long_t now, I don't see why this cannot be replaced with an atomic_t.

Comments?

Index: linux-2.6.16-rc1/include/linux/percpu_counter.h
===================================================================
--- linux-2.6.16-rc1.orig/include/linux/percpu_counter.h	2006-01-24 13:52:24.000000000 -0800
+++ linux-2.6.16-rc1/include/linux/percpu_counter.h	2006-01-24 14:10:26.000000000 -0800
@@ -15,8 +15,7 @@
 #ifdef CONFIG_SMP
 
 struct percpu_counter {
-	spinlock_t lock;
-	long count;
+	atomic_long_t count;
 	long *counters;
 };
 
@@ -28,8 +27,7 @@ struct percpu_counter {
 
 static inline void percpu_counter_init(struct percpu_counter *fbc)
 {
-	spin_lock_init(&fbc->lock);
-	fbc->count = 0;
+	atomic_long_set(&fbc->count, 0);
 	fbc->counters = alloc_percpu(long);
 }
 
@@ -42,7 +40,7 @@ void percpu_counter_mod(struct percpu_co
 
 static inline long percpu_counter_read(struct percpu_counter *fbc)
 {
-	return fbc->count;
+	return atomic_long_read(&fbc->count);
 }
 
 /*
@@ -51,7 +49,7 @@ static inline long percpu_counter_read(s
  */
 static inline long percpu_counter_read_positive(struct percpu_counter *fbc)
 {
-	long ret = fbc->count;
+	long ret = atomic_long_read(&fbc->count);
 
 	barrier();		/* Prevent reloads of fbc->count */
 	if (ret > 0)
Index: linux-2.6.16-rc1/mm/swap.c
===================================================================
--- linux-2.6.16-rc1.orig/mm/swap.c	2006-01-17 14:12:17.000000000 -0800
+++ linux-2.6.16-rc1/mm/swap.c	2006-01-24 14:23:20.000000000 -0800
@@ -449,9 +449,7 @@ void percpu_counter_mod(struct percpu_co
 	pcount = per_cpu_ptr(fbc->counters, cpu);
 	count = *pcount + amount;
 	if (count >= FBC_BATCH || count <= -FBC_BATCH) {
-		spin_lock(&fbc->lock);
-		fbc->count += count;
-		spin_unlock(&fbc->lock);
+		atomic_long_add(count, &fbc->count);
 		count = 0;
 	}
 	*pcount = count;