Messages in this thread | | | Subject | Re: [PATCH] sched/fair: fix mul overflow on 32-bit systems | From | Andrey Ryabinin <> | Date | Fri, 11 Dec 2015 17:00:01 +0300 |
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On 12/11/2015 04:36 PM, Peter Zijlstra wrote: > On Fri, Dec 11, 2015 at 02:25:51PM +0100, Peter Zijlstra wrote: >> On Fri, Dec 11, 2015 at 03:55:18PM +0300, Andrey Ryabinin wrote: >>> Make 'r' 64-bit type to avoid overflow in 'r * LOAD_AVG_MAX' >>> on 32-bit systems: >>> UBSAN: Undefined behaviour in kernel/sched/fair.c:2785:18 >>> signed integer overflow: >>> 87950 * 47742 cannot be represented in type 'int' >>> >>> Fixes: 9d89c257dfb9 ("sched/fair: Rewrite runnable load and utilization average tracking") >>> Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com> >>> --- >>> kernel/sched/fair.c | 4 ++-- >>> 1 file changed, 2 insertions(+), 2 deletions(-) >>> >>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c >>> index e3266eb..733f0b8 100644 >>> --- a/kernel/sched/fair.c >>> +++ b/kernel/sched/fair.c >>> @@ -2780,14 +2780,14 @@ static inline int update_cfs_rq_load_avg(u64 now, struct cfs_rq *cfs_rq) >>> int decayed, removed = 0; >>> >>> if (atomic_long_read(&cfs_rq->removed_load_avg)) { >>> - long r = atomic_long_xchg(&cfs_rq->removed_load_avg, 0); >>> + s64 r = atomic_long_xchg(&cfs_rq->removed_load_avg, 0); >>> sa->load_avg = max_t(long, sa->load_avg - r, 0); >>> sa->load_sum = max_t(s64, sa->load_sum - r * LOAD_AVG_MAX, 0); >> >> This makes sense, because sched_avg::load_sum is u64. >> >>> removed = 1; >>> } >>> >>> if (atomic_long_read(&cfs_rq->removed_util_avg)) { >>> - long r = atomic_long_xchg(&cfs_rq->removed_util_avg, 0); >>> + s64 r = atomic_long_xchg(&cfs_rq->removed_util_avg, 0); >>> sa->util_avg = max_t(long, sa->util_avg - r, 0); >>> sa->util_sum = max_t(s32, sa->util_sum - r * LOAD_AVG_MAX, 0); >>> } >> >> However sched_avg::util_sum is u32, so this is still wrecked. > > I seems to have wrecked that in: > > 006cdf025a33 ("sched/fair: Optimize per entity utilization tracking") > > maybe just make util_load u64 too? >
Is there any guarantee that the final result of expression 'util_sum - r * LOAD_AVG_MAX' always can be represented by s32?
If yes, than we could just do this: max_t(s32, (u64)sa->util_sum - r * LOAD_AVG_MAX, 0)
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