fs: Use this_cpu_xx operations in buffer.c

Optimize various per cpu area operations through these new percpu
operations.  These operations avoid address calculations through the
use of segment prefixes and multiple memory references through RMW
instructions etc.

Reduces code size:

Before:

christoph@linux-2.6$ size fs/buffer.o
   text	   data	    bss	    dec	    hex	filename
  19169	     80	     28	  19277	   4b4d	fs/buffer.o

After:

christoph@linux-2.6$ size fs/buffer.o
   text	   data	    bss	    dec	    hex	filename
  19138	     80	     28	  19246	   4b2e	fs/buffer.o

V3->V4:
	- Move the use of this_cpu_inc_return into a later patch so that
	  this one can go in without percpu infrastructure changes.

Cc: Wu Fengguang <fengguang.wu@intel.com>
Cc: Christoph Hellwig <hch@lst.de>
Acked-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

fs/buffer.c

@@ -1270,12 +1270,10 @@ static inline void check_irqs_on(void)
 static void bh_lru_install(struct buffer_head *bh)
 {
 	struct buffer_head *evictee = NULL;
-	struct bh_lru *lru;
 
 	check_irqs_on();
 	bh_lru_lock();
-	lru = &__get_cpu_var(bh_lrus);
-	if (lru->bhs[0] != bh) {
+	if (__this_cpu_read(bh_lrus.bhs[0]) != bh) {
 		struct buffer_head *bhs[BH_LRU_SIZE];
 		int in;
 		int out = 0;
@@ -1283,7 +1281,8 @@ static void bh_lru_install(struct buffer_head *bh)
 		get_bh(bh);
 		bhs[out++] = bh;
 		for (in = 0; in < BH_LRU_SIZE; in++) {
-			struct buffer_head *bh2 = lru->bhs[in];
+			struct buffer_head *bh2 =
+				__this_cpu_read(bh_lrus.bhs[in]);
 
 			if (bh2 == bh) {
 				__brelse(bh2);
@@ -1298,7 +1297,7 @@ static void bh_lru_install(struct buffer_head *bh)
 		}
 		while (out < BH_LRU_SIZE)
 			bhs[out++] = NULL;
-		memcpy(lru->bhs, bhs, sizeof(bhs));
+		memcpy(__this_cpu_ptr(&bh_lrus.bhs), bhs, sizeof(bhs));
 	}
 	bh_lru_unlock();
 
@@ -1313,23 +1312,22 @@ static struct buffer_head *
 lookup_bh_lru(struct block_device *bdev, sector_t block, unsigned size)
 {
 	struct buffer_head *ret = NULL;
-	struct bh_lru *lru;
 	unsigned int i;
 
 	check_irqs_on();
 	bh_lru_lock();
-	lru = &__get_cpu_var(bh_lrus);
 	for (i = 0; i < BH_LRU_SIZE; i++) {
-		struct buffer_head *bh = lru->bhs[i];
+		struct buffer_head *bh = __this_cpu_read(bh_lrus.bhs[i]);
 
 		if (bh && bh->b_bdev == bdev &&
 				bh->b_blocknr == block && bh->b_size == size) {
 			if (i) {
 				while (i) {
-					lru->bhs[i] = lru->bhs[i - 1];
+					__this_cpu_write(bh_lrus.bhs[i],
+						__this_cpu_read(bh_lrus.bhs[i - 1]));
 					i--;
 				}
-				lru->bhs[0] = bh;
+				__this_cpu_write(bh_lrus.bhs[0], bh);
 			}
 			get_bh(bh);
 			ret = bh;
@@ -3205,20 +3203,21 @@ static void recalc_bh_state(void)
 
 	if (__get_cpu_var(bh_accounting).ratelimit++ < 4096)
 		return;
-	__get_cpu_var(bh_accounting).ratelimit = 0;
+	__this_cpu_write(bh_accounting.ratelimit, 0);
 	for_each_online_cpu(i)
 		tot += per_cpu(bh_accounting, i).nr;
 	buffer_heads_over_limit = (tot > max_buffer_heads);
 }
-	
+
 struct buffer_head *alloc_buffer_head(gfp_t gfp_flags)
 {
 	struct buffer_head *ret = kmem_cache_zalloc(bh_cachep, gfp_flags);
 	if (ret) {
 		INIT_LIST_HEAD(&ret->b_assoc_buffers);
-		get_cpu_var(bh_accounting).nr++;
+		preempt_disable();
+		__this_cpu_inc(bh_accounting.nr);
 		recalc_bh_state();
-		put_cpu_var(bh_accounting);
+		preempt_enable();
 	}
 	return ret;
 }
@@ -3228,9 +3227,10 @@ void free_buffer_head(struct buffer_head *bh)
 {
 	BUG_ON(!list_empty(&bh->b_assoc_buffers));
 	kmem_cache_free(bh_cachep, bh);
-	get_cpu_var(bh_accounting).nr--;
+	preempt_disable();
+	__this_cpu_dec(bh_accounting.nr);
 	recalc_bh_state();
-	put_cpu_var(bh_accounting);
+	preempt_enable();
 }
 EXPORT_SYMBOL(free_buffer_head);
 
@@ -3243,9 +3243,8 @@ static void buffer_exit_cpu(int cpu)
 		brelse(b->bhs[i]);
 		b->bhs[i] = NULL;
 	}
-	get_cpu_var(bh_accounting).nr += per_cpu(bh_accounting, cpu).nr;
+	this_cpu_add(bh_accounting.nr, per_cpu(bh_accounting, cpu).nr);
 	per_cpu(bh_accounting, cpu).nr = 0;
-	put_cpu_var(bh_accounting);
 }
 
 static int buffer_cpu_notify(struct notifier_block *self,