u64_stats: provide u64_stats_t type

On 64bit arches, struct u64_stats_sync is empty and provides
no help against load/store tearing.

Using READ_ONCE()/WRITE_ONCE() would be needed.

But the update side would be slightly more expensive.

local64_t was defined so that we could use regular adds
in a manner which is atomic wrt IRQs.

However the u64_stats infra means we do not have to use
local64_t on 32bit arches since the syncp provides the needed
protection.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/u64_stats_sync.h

@@ -40,8 +40,8 @@
  *   spin_lock_bh(...) or other synchronization to get exclusive access
  *   ...
  *   u64_stats_update_begin(&stats->syncp);
- *   stats->bytes64 += len; // non atomic operation
- *   stats->packets64++;    // non atomic operation
+ *   u64_stats_add(&stats->bytes64, len); // non atomic operation
+ *   u64_stats_inc(&stats->packets64);    // non atomic operation
  *   u64_stats_update_end(&stats->syncp);
  *
  * While a consumer (reader) should use following template to get consistent
@@ -52,8 +52,8 @@
  *
  * do {
  *         start = u64_stats_fetch_begin(&stats->syncp);
- *         tbytes = stats->bytes64; // non atomic operation
- *         tpackets = stats->packets64; // non atomic operation
+ *         tbytes = u64_stats_read(&stats->bytes64); // non atomic operation
+ *         tpackets = u64_stats_read(&stats->packets64); // non atomic operation
  * } while (u64_stats_fetch_retry(&stats->syncp, start));
  *
  *
@@ -68,6 +68,49 @@ struct u64_stats_sync {
 #endif
 };
 
+#if BITS_PER_LONG == 64
+#include <asm/local64.h>
+
+typedef struct {
+	local64_t	v;
+} u64_stats_t ;
+
+static inline u64 u64_stats_read(const u64_stats_t *p)
+{
+	return local64_read(&p->v);
+}
+
+static inline void u64_stats_add(u64_stats_t *p, unsigned long val)
+{
+	local64_add(val, &p->v);
+}
+
+static inline void u64_stats_inc(u64_stats_t *p)
+{
+	local64_inc(&p->v);
+}
+
+#else
+
+typedef struct {
+	u64		v;
+} u64_stats_t;
+
+static inline u64 u64_stats_read(const u64_stats_t *p)
+{
+	return p->v;
+}
+
+static inline void u64_stats_add(u64_stats_t *p, unsigned long val)
+{
+	p->v += val;
+}
+
+static inline void u64_stats_inc(u64_stats_t *p)
+{
+	p->v++;
+}
+#endif
 
 static inline void u64_stats_init(struct u64_stats_sync *syncp)
 {