Messages in this thread | | | Date | Tue, 27 Jun 2023 00:46:05 +0200 | From | Frederic Weisbecker <> | Subject | Re: [patch 09/45] posix-cpu-timers: Fix posix_cpu_timer_get() behaviour |
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On Tue, Jun 06, 2023 at 04:37:33PM +0200, Thomas Gleixner wrote: > timer_gettime() must return the remaining time to the next expiry of a > timer or 0 if the timer is not armed and no signal pending. > > This has to be correct also for interval timers even if the signal is > pending or the timer has been created with SIGEV_NONE. > > The posix CPU timer implementation fails for both cases as it does not > forward the timer in posix_cpu_timer_get() before calculating the expiry > time. > > It neither clears the expiry value when a oneshot SIGEV_NONE timer expired > and returns 1nsec instead, which is only correct for timers with signals > when the signal delivery did not happen yet. > > 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...
> > Make the update logic a separate function so it can be reused to simplify > posix_cpu_timer_set(). > > Fixes: ae1a78eecc45 ("cpu-timers: Return correct previous timer reload value") > Signed-off-by: Thomas Gleixner <tglx@linutronix.de> > --- > kernel/time/posix-cpu-timers.c | 96 +++++++++++++++++++++++------------------ > 1 file changed, 54 insertions(+), 42 deletions(-) > > --- a/kernel/time/posix-cpu-timers.c > +++ b/kernel/time/posix-cpu-timers.c > @@ -785,45 +782,60 @@ static int posix_cpu_timer_set(struct k_ > return ret; > } > > -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.
Also we are not holding the sighand lock here. So even though the timer is forwarded, it may still be picked up afterward by check_thread_timers() based on its stalled previous expires value... This can create a discrepancy between the overrun count and the actual events received, and perhaps other funny things...
Thanks.
> > /* > - * Sample the clock to take the difference with the expiry time. > + * Interval timers cannot have a remaining time <= 0 because the > + * forwarding guarantees to move them forward so that the next > + * timer expiry is > @now. > */ > - if (CPUCLOCK_PERTHREAD(timer->it_clock)) > - now = cpu_clock_sample(clkid, p); > - else > - now = cpu_clock_sample_group(clkid, p, false); > - > if (now < expires) { > itp->it_value = ns_to_timespec64(expires - now); > } else { > /* > - * The timer should have expired already, but the firing > - * hasn't taken place yet. Say it's just about to expire. > + * A single shot SIGEV_NONE timer must return 0, when it is > + * expired! Timers which have a real signal delivery mode > + * must return a remaining time greater than 0 because the > + * signal has not yet been delivered. > */ > - itp->it_value.tv_nsec = 1; > - itp->it_value.tv_sec = 0; > + if (!sigev_none) > + itp->it_value.tv_nsec = 1; > + } > +}
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