sysctl: Fix data races in proc_dointvec().

A sysctl variable is accessed concurrently, and there is always a chance
of data-race.  So, all readers and writers need some basic protection to
avoid load/store-tearing.

This patch changes proc_dointvec() to use READ_ONCE() and WRITE_ONCE()
internally to fix data-races on the sysctl side.  For now, proc_dointvec()
itself is tolerant to a data-race, but we still need to add annotations on
the other subsystem's side.

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

kernel/sysctl.c

@@ -446,14 +446,14 @@ static int do_proc_dointvec_conv(bool *negp, unsigned long *lvalp,
 		if (*negp) {
 			if (*lvalp > (unsigned long) INT_MAX + 1)
 				return -EINVAL;
-			*valp = -*lvalp;
+			WRITE_ONCE(*valp, -*lvalp);
 		} else {
 			if (*lvalp > (unsigned long) INT_MAX)
 				return -EINVAL;
-			*valp = *lvalp;
+			WRITE_ONCE(*valp, *lvalp);
 		}
 	} else {
-		int val = *valp;
+		int val = READ_ONCE(*valp);
 		if (val < 0) {
 			*negp = true;
 			*lvalp = -(unsigned long)val;