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;
-
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