sysctl: Fix data races in proc_dointvec_jiffies().

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_jiffies() to use READ_ONCE() and
WRITE_ONCE() internally to fix data-races on the sysctl side.  For now,
proc_dointvec_jiffies() 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

@@ -1173,9 +1173,12 @@ static int do_proc_dointvec_jiffies_conv(bool *negp, unsigned long *lvalp,
 	if (write) {
 		if (*lvalp > INT_MAX / HZ)
 			return 1;
-		*valp = *negp ? -(*lvalp*HZ) : (*lvalp*HZ);
+		if (*negp)
+			WRITE_ONCE(*valp, -*lvalp * HZ);
+		else
+			WRITE_ONCE(*valp, *lvalp * HZ);
 	} else {
-		int val = *valp;
+		int val = READ_ONCE(*valp);
 		unsigned long lval;
 		if (val < 0) {
 			*negp = true;