staging: zcache: fix serialization bug in zv stats

In a multithreaded workload, the zv_curr_dist_counts
and zv_cumul_dist_counts statistics are being corrupted
because the increments and decrements in zv_create
and zv_free are not atomic.

This patch converts these statistics and their corresponding
increments/decrements/reads to atomic operations.

Signed-off-by: Seth Jennings <sjenning@linux.vnet.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/zcache/zcache-main.c

@@ -690,8 +690,8 @@ static unsigned int zv_max_zsize = (PAGE_SIZE / 8) * 7;
  */
 static unsigned int zv_max_mean_zsize = (PAGE_SIZE / 8) * 5;
 
-static unsigned long zv_curr_dist_counts[NCHUNKS];
-static unsigned long zv_cumul_dist_counts[NCHUNKS];
+static atomic_t zv_curr_dist_counts[NCHUNKS];
+static atomic_t zv_cumul_dist_counts[NCHUNKS];
 
 static struct zv_hdr *zv_create(struct xv_pool *xvpool, uint32_t pool_id,
 				struct tmem_oid *oid, uint32_t index,
@@ -710,8 +710,8 @@ static struct zv_hdr *zv_create(struct xv_pool *xvpool, uint32_t pool_id,
 			&page, &offset, ZCACHE_GFP_MASK);
 	if (unlikely(ret))
 		goto out;
-	zv_curr_dist_counts[chunks]++;
-	zv_cumul_dist_counts[chunks]++;
+	atomic_inc(&zv_curr_dist_counts[chunks]);
+	atomic_inc(&zv_cumul_dist_counts[chunks]);
 	zv = kmap_atomic(page, KM_USER0) + offset;
 	zv->index = index;
 	zv->oid = *oid;
@@ -733,7 +733,7 @@ static void zv_free(struct xv_pool *xvpool, struct zv_hdr *zv)
 
 	ASSERT_SENTINEL(zv, ZVH);
 	BUG_ON(chunks >= NCHUNKS);
-	zv_curr_dist_counts[chunks]--;
+	atomic_dec(&zv_curr_dist_counts[chunks]);
 	size -= sizeof(*zv);
 	BUG_ON(size == 0);
 	INVERT_SENTINEL(zv, ZVH);
@@ -773,7 +773,7 @@ static int zv_curr_dist_counts_show(char *buf)
 	char *p = buf;
 
 	for (i = 0; i < NCHUNKS; i++) {
-		n = zv_curr_dist_counts[i];
+		n = atomic_read(&zv_curr_dist_counts[i]);
 		p += sprintf(p, "%lu ", n);
 		chunks += n;
 		sum_total_chunks += i * n;
@@ -789,7 +789,7 @@ static int zv_cumul_dist_counts_show(char *buf)
 	char *p = buf;
 
 	for (i = 0; i < NCHUNKS; i++) {
-		n = zv_cumul_dist_counts[i];
+		n = atomic_read(&zv_cumul_dist_counts[i]);
 		p += sprintf(p, "%lu ", n);
 		chunks += n;
 		sum_total_chunks += i * n;