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

From: Mike Galbraith
Date: Tue Mar 04 2014 - 00:38:33 EST


(crap crap crap... M.A.I.N.T.A.I.N.E.R.S _dummy_)

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 <john.stultz@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);
}

/**




--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/