This Linux kernel change "gfs2: Fix sign extension bug in gfs2_update_stats" is included in the Linux 5.2 release. This change is authored by Andreas Gruenbacher <agruenba [at] redhat.com> on Fri May 17 19:18:43 2019 +0100. The commit for this change in Linux stable tree is 5a5ec83 (patch).
gfs2: Fix sign extension bug in gfs2_update_stats Commit 4d207133e9c3 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: 4d207133e9c3 ("gfs2: Make statistics unsigned, suitable for use with do_div()") Signed-off-by: Andreas Gruenbacher <[email protected]> Cc: [email protected] # v4.4+
There are 9 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.
fs/gfs2/lock_dlm.c | 9 +++++---- 1 file changed, 5 insertions(+), 4 deletions(-) diff --git a/fs/gfs2/lock_dlm.c b/fs/gfs2/lock_dlm.c index 31df26e..69bd159 100644 --- a/fs/gfs2/lock_dlm.c +++ b/fs/gfs2/lock_dlm.c @@ -31,9 +31,10 @@ * @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 @@ -49,7 +50,7 @@ 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; } /**