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SubjectRe: [patch 09/45] posix-cpu-timers: Fix posix_cpu_timer_get() behaviour
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On Tue, Jun 27 2023 at 00:46, Frederic Weisbecker wrote:> On Tue, Jun 06, 2023 at 04:37:33PM +0200, Thomas Gleixner wrote:
>> Aside of that posix_cpu_timer_set() pointlessly arms SIGEV_NONE timers
>> which are later disarmed when the initial expiry happens. That's bogus and
>> just keeping the process wide timer active for nothing.
>>
>> Cure this by:
>>
>> 1) Avoiding to arm SIGEV_NONE timers
>>
>> 2) Forwarding interval timers in posix_cpu_timer_get()
>>
>> 3) Taking SIGEV_NONE into account when a oneshot timer has expired
>
> This patch does too many things at once...

Let me try again to split it up. I failed before ...

>> -static void posix_cpu_timer_get(struct k_itimer *timer, struct itimerspec64 *itp)
>> +static void __posix_cpu_timer_get(struct k_itimer *timer, struct itimerspec64 *itp, u64 now)
>> {
>> - clockid_t clkid = CPUCLOCK_WHICH(timer->it_clock);
>> - struct cpu_timer *ctmr = &timer->it.cpu;
>> - u64 now, expires = cpu_timer_getexpires(ctmr);
>> - struct task_struct *p;
>> -
>> - rcu_read_lock();
>> - p = cpu_timer_task_rcu(timer);
>> - if (!p)
>> - goto out;
>> + bool sigev_none = timer->it_sigev_notify == SIGEV_NONE;
>> + u64 expires;
>>
>> /*
>> - * Easy part: convert the reload time.
>> + * Make sure that interval timers are moved forward for the
>> + * following cases:
>> + * - SIGEV_NONE timers which are never armed
>> + * - Timers which expired, but the signal has not yet been
>> + * delivered
>> */
>> - itp->it_interval = ktime_to_timespec64(timer->it_interval);
>> -
>> - if (!expires)
>> - goto out;
>> + expires = bump_cpu_timer(timer, now);
>
> What if the expiration has been reached but we arrived here before
> handle_posix_cpu_timers() had a chance? In that case the call to
> bump_cpu_timer() may forward the timer and artificially create an
> overrun / missed event.

Bah. This clearly misses some conditionals so that it actually handles
the cases described in the comment above it...

Thanks,

tglx


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