[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>

1 files changed, 7 insertions, 1 deletions

diff --git a/arch/s390/kernel/time.c b/arch/s390/kernel/time.c
index 9a1d95894f3d..c36353e8c140 100644
--- a/arch/s390/kernel/time.c
+++ b/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)) {