proc: loadavg reading race

The avenrun[] values are supposed to be protected by xtime_lock.
loadavg_read_proc does not use it.  Theoretically this may result in an
occasional glitch when the value read from /proc/loadavg would be as much
as 1<<11 times higher than it should be.
Signed-off-by: NMichal Schmidt <mschmidt@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

fs/proc/proc_misc.c

@@ -84,10 +84,15 @@ static int loadavg_read_proc(char *page, char **start, off_t off,
 {
 	int a, b, c;
 	int len;
+	unsigned long seq;
+
+	do {
+		seq = read_seqbegin(&xtime_lock);
+		a = avenrun[0] + (FIXED_1/200);
+		b = avenrun[1] + (FIXED_1/200);
+		c = avenrun[2] + (FIXED_1/200);
+	} while (read_seqretry(&xtime_lock, seq));
 
-	a = avenrun[0] + (FIXED_1/200);
-	b = avenrun[1] + (FIXED_1/200);
-	c = avenrun[2] + (FIXED_1/200);
 	len = sprintf(page,"%d.%02d %d.%02d %d.%02d %ld/%d %d\n",
 		LOAD_INT(a), LOAD_FRAC(a),
 		LOAD_INT(b), LOAD_FRAC(b),