[PATCH] s390: stop_hz_timer vs. xtime updates

The calculation of the value return by next_timer_interrupt from jiffies to
jiffies_64 is racy against xtime updates.  We need to protect the calculation
with read_seqbegin/read_seqretry.
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

arch/s390/kernel/time.c

@@ -237,6 +237,8 @@ int sysctl_hz_timer = 1;
  */
 static inline void stop_hz_timer(void)
 {
+	unsigned long flags;
+	unsigned long seq, next;
 	__u64 timer, todval;
 
 	if (sysctl_hz_timer != 0)
@@ -257,7 +259,11 @@ static inline void stop_hz_timer(void)
 	 * This cpu is going really idle. Set up the clock comparator
 	 * for the next event.
 	 */
-	timer = (__u64) (next_timer_interrupt() - jiffies) + jiffies_64;
+	next = next_timer_interrupt();
+	do {
+		seq = read_seqbegin_irqsave(&xtime_lock, flags);
+		timer = (__u64)(next - jiffies) + jiffies_64;
+	} while (read_seqretry_irqrestore(&xtime_lock, seq, flags));
 	todval = -1ULL;
 	/* Be careful about overflows. */
 	if (timer < (-1ULL / CLK_TICKS_PER_JIFFY)) {