[RFC][PATCH] clocksource: avoid unnecessary overflow in cyclecounter_cyc2ns()

[Mike Galbraith]

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 <sqazi@xxxxxxxxxx>
Cc: John Stultz <johnstul@xxxxxxxxxx>
Signed-off-by: Mike Galbraith <bitbucket@xxxxxxxxx>
---
 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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