[PATCH] s390: next_timer_interrupt overflow in stop_hz_timer

The 32 bit unsigned substraction (next - jiffies) in stop_hz_timer can
overflow if jiffies gets advanced between next_timer_interrupt and the read
under the xtime lock.  The cast to a u64 then results in a large value
which causes the cpu to wait too long.  Fix this by casting next and
jiffies independently to u64 before subtracting them.

(Spotted by Zachary Amsden <[email protected]>)

Signed-off-by: Martin Schwidefsky <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

arch/s390/kernel/time.c

@@ -272,7 +272,7 @@ static inline void stop_hz_timer(void)
 	next = next_timer_interrupt();
 	do {
 		seq = read_seqbegin_irqsave(&xtime_lock, flags);
-		timer = (__u64)(next - jiffies) + jiffies_64;
+		timer = (__u64 next) - (__u64 jiffies) + jiffies_64;
 	} while (read_seqretry_irqrestore(&xtime_lock, seq, flags));
 	todval = -1ULL;
 	/* Be careful about overflows. */