[PATCH 3/5] mm/vmstat: use cmpxchg loop in cpu_vm_stats_fold

[Marcelo Tosatti <mtosatti@xxxxxxxxxx>]

In preparation to switch vmstat shepherd to flush
per-CPU counters remotely, use a cmpxchg loop 
instead of a pair of read/write instructions.

Signed-off-by: Marcelo Tosatti <mtosatti@xxxxxxxxxx>

Index: linux-vmstat-remote/mm/vmstat.c
===================================================================
--- linux-vmstat-remote.orig/mm/vmstat.c
+++ linux-vmstat-remote/mm/vmstat.c
@@ -885,7 +885,7 @@ static int refresh_cpu_vm_stats(void)
 }
 
 /*
- * Fold the data for an offline cpu into the global array.
+ * Fold the data for a cpu into the global array.
  * There cannot be any access by the offline cpu and therefore
  * synchronization is simplified.
  */
@@ -906,8 +906,9 @@ void cpu_vm_stats_fold(int cpu)
 			if (pzstats->vm_stat_diff[i]) {
 				int v;
 
-				v = pzstats->vm_stat_diff[i];
-				pzstats->vm_stat_diff[i] = 0;
+				do {
+					v = pzstats->vm_stat_diff[i];
+				} while (cmpxchg(&pzstats->vm_stat_diff[i], v, 0) != v);
 				atomic_long_add(v, &zone->vm_stat[i]);
 				global_zone_diff[i] += v;
 			}
@@ -917,8 +918,9 @@ void cpu_vm_stats_fold(int cpu)
 			if (pzstats->vm_numa_event[i]) {
 				unsigned long v;
 
-				v = pzstats->vm_numa_event[i];
-				pzstats->vm_numa_event[i] = 0;
+				do {
+					v = pzstats->vm_numa_event[i];
+				} while (cmpxchg(&pzstats->vm_numa_event[i], v, 0) != v);
 				zone_numa_event_add(v, zone, i);
 			}
 		}
@@ -934,8 +936,9 @@ void cpu_vm_stats_fold(int cpu)
 			if (p->vm_node_stat_diff[i]) {
 				int v;
 
-				v = p->vm_node_stat_diff[i];
-				p->vm_node_stat_diff[i] = 0;
+				do {
+					v = p->vm_node_stat_diff[i];
+				} while (cmpxchg(&p->vm_node_stat_diff[i], v, 0) != v);
 				atomic_long_add(v, &pgdat->vm_stat[i]);
 				global_node_diff[i] += v;
 			}