sysctl: Fix data races in proc_doulongvec_minmax().

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

Fixes: 1da177e4c3 ("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

@@ -1090,9 +1090,9 @@ static int __do_proc_doulongvec_minmax(void *data, struct ctl_table *table,
 				err = -EINVAL;
 				break;
 			}
-			*i = val;
+			WRITE_ONCE(*i, val);
 		} else {
-			val = convdiv * (*i) / convmul;
+			val = convdiv * READ_ONCE(*i) / convmul;
 			if (!first)
 				proc_put_char(&buffer, &left, '\t');
 			proc_put_long(&buffer, &left, val, false);