Re: [PATCH v3 1/2] timers: Fix usleep_range() in the context of wake_up_process()

Thomas Gleixner
2016-10-20T21:25:38Z

On Thu, 20 Oct 2016, Douglas Anderson wrote:
> - An effort was made to look for users relying on the old behavior by
> looking for usleep_range() in the same file as wake_up_process().
> No problems was found by this search, though it is conceivable that
> someone could have put the sleep and wakeup in two different files.
> - An effort was made to ask several upstream maintainers if they were
> aware of people relying on wake_up_process() to wake up
> usleep_range(). No maintainers were aware of that but they were aware
> of many people relying on usleep_range() never returning before the
> minimum.

Thanks for going the extra mile !

> static void __sched do_usleep_range(unsigned long min, unsigned long max)
> {
> + ktime_t now, end;
> ktime_t kmin;
> u64 delta;
> + int ret;
>
> - kmin = ktime_set(0, min * NSEC_PER_USEC);
> + now = ktime_get();
> + end = ktime_add_us(now, min);

So you calculate the absolute expiry time here.

> delta = (u64)(max - min) * NSEC_PER_USEC;
> - schedule_hrtimeout_range(&kmin, delta, HRTIMER_MODE_REL);
> + do {
> + kmin = ktime_sub(end, now);
> + ret = schedule_hrtimeout_range(&kmin, delta, HRTIMER_MODE_REL);

And then you schedule the thing relative. That does not make sense.

> +
> + /*
> + * If schedule_hrtimeout_range() returns 0 then we actually
> + * hit the timeout. If not then we need to re-calculate the
> + * new timeout ourselves.
> + */
> + if (ret == 0)
> + break;
> +
> + now = ktime_get();

And this is broken because schedule_hrtimeout_range() returns with task
state TASK_RUNNING so the next schedule_hrtimeout_range() will return
-EINTR again because nothing sets the task state back to UNINTERRUPTIBLE.
So instead of sleeping you busy loop.

What you really want to do is something like this:

void __sched usleep_range(unsigned long min, unsigned long max)
{
ktime_t expires = ktime_add_us(ktime_get(), min * NSEC_PER_USEC);
ktime_t delta = ktime_set(0, (u64)(max - min) * NSEC_PER_USEC);

for (;;) {
__set_current_state(TASK_UNINTERRUPTIBLE);
/* Do not return before the requested sleep time has elapsed */
if (!schedule_hrtimeout_range(&expires, delta, HRTIMER_MODE_ABS))
break;
}
}

Thanks,

tglx