Greetings,
While rummaging around looking for HTH a gaggle of weird a$$ machines
can manage to timewarp back and forth by exactly 208 days, I stumbled
across $subject which looks like it may want to borrow Salman's fix.
clocksource: avoid unnecessary overflow in cyclecounter_cyc2ns()
As per 4cecf6d401a "sched, x86: Avoid unnecessary overflow in sched_clock",
cycles * mult >> shift is overflow prone. so give it the same treatment.
Cc: Salman Qazi <sq...@google.com>
Cc: John Stultz <johns...@us.ibm.com>
Signed-off-by: Mike Galbraith <bitbuc...@online.de>
---
include/linux/clocksource.h | 11 ++++++++---
1 file changed, 8 insertions(+), 3 deletions(-)
--- a/include/linux/clocksource.h
+++ b/include/linux/clocksource.h
@@ -77,13 +77,18 @@ struct timecounter {
*
* XXX - This could use some mult_lxl_ll() asm optimization. Same code
* as in cyc2ns, but with unsigned result.
+ *
+ * Because it is the same as x86 __cycles_2_ns, give it the same treatment as
+ * commit 4cecf6d401a "sched, x86: Avoid unnecessary overflow in sched_clock"
+ * to avoid a potential cycles * mult overflow.
*/
static inline u64 cyclecounter_cyc2ns(const struct cyclecounter *cc,
cycle_t cycles)
{
- u64 ret = (u64)cycles;
- ret = (ret * cc->mult) >> cc->shift;
- return ret;
+ u64 quot = (u64)cycles >> cc->shift;
+ u64 rem = (u64)cycles & ((1ULL << cc->shift) - 1);
+
+ return quot * cc->mult + ((rem * cc->mult) >> cc->shift);
}
/**
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