gfs2: Fix sign extension bug in gfs2_update_stats

BugLink: https://bugs.launchpad.net/bugs/1832661

commit 5a5ec83d upstream.

Commit 4d207133 changed the types of the statistic values in struct
gfs2_lkstats from s64 to u64.  Because of that, what should be a signed
value in gfs2_update_stats turned into an unsigned value.  When shifted
right, we end up with a large positive value instead of a small negative
value, which results in an incorrect variance estimate.

Fixes: 4d207133 ("gfs2: Make statistics unsigned, suitable for use with do_div()")
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Cc: stable@vger.kernel.org # v4.4+
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

fs/gfs2/lock_dlm.c

@@ -32,9 +32,10 @@ extern struct workqueue_struct *gfs2_control_wq;
  * @delta is the difference between the current rtt sample and the
  * running average srtt. We add 1/8 of that to the srtt in order to
  * update the current srtt estimate. The variance estimate is a bit
- * more complicated. We subtract the abs value of the @delta from
- * the current variance estimate and add 1/4 of that to the running
- * total.
+ * more complicated. We subtract the current variance estimate from
+ * the abs value of the @delta and add 1/4 of that to the running
+ * total.  That's equivalent to 3/4 of the current variance
+ * estimate plus 1/4 of the abs of @delta.
  *
  * Note that the index points at the array entry containing the smoothed
  * mean value, and the variance is always in the following entry
@@ -50,7 +51,7 @@ static inline void gfs2_update_stats(struct gfs2_lkstats *s, unsigned index,
 	s64 delta = sample - s->stats[index];
 	s->stats[index] += (delta >> 3);
 	index++;
-	s->stats[index] += ((abs(delta) - s->stats[index]) >> 2);
+	s->stats[index] += (s64)(abs(delta) - s->stats[index]) >> 2;
 }
 
 /**