linux-kernel - Re: [patch V2 07/50] posix-cpu-timers: Split up posix_cpu_timer

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Date: Fri, 12 Apr 2024 13:25:01 -0500
From: "Eric W. Biederman" <ebiederm@...ssion.com>
To: Thomas Flexing <tglx@...utronix.de>
Cc: LKML <linux-kernel@...r.kernel.org>,  Anna-Maria Behnsen
 <anna-maria@...utronix.de>,  Frederic Weisbecker <frederic@...nel.org>,
  John Stultz <jstultz@...gle.com>,  Peter Zijlstra <peterz@...radead.org>,
  Ingo Molnar <mingo@...nel.org>,  Stephen Boyd <sboyd@...nel.org>,  Oleg
 Nesterov <oleg@...hat.com>
Subject: Re: [patch V2 07/50] posix-cpu-timers: Split up posix_cpu_timer_get()

Thomas Gleixner <tglx@...utronix.de> writes:

> In preparation for addressing issues in the timer_get() and timer_set()
> functions of posix CPU timers.

To see that this was safe I had to lookup and see that
cpu_timer_getexpires is a truly trivial function.

static inline u64 cpu_timer_getexpires(struct cpu_timer *ctmr)
{
	return ctmr->node.expires;
}

I am a bit confused by the purpose of this function in
posix-cpu-timers.c.  In some places this helper is used (like below),
and in other places like bump_cpu_timer the expires member is
accessed directly.

It isn't really a problem, but it is something that might be
worth making consistent in the code to make it easier to read.

Eric

> No functional change.
>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> ---
> V2: Split out into new patch to make review simpler - Frederic
> ---
>  kernel/time/posix-cpu-timers.c |   51 +++++++++++++++++++----------------------
>  1 file changed, 24 insertions(+), 27 deletions(-)
>
> --- a/kernel/time/posix-cpu-timers.c
> +++ b/kernel/time/posix-cpu-timers.c
> @@ -785,33 +785,9 @@ 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;
> -
> -	/*
> -	 * Easy part: convert the reload time.
> -	 */
> -	itp->it_interval = ktime_to_timespec64(timer->it_interval);
> -
> -	if (!expires)
> -		goto out;
> -
> -	/*
> -	 * Sample the clock to take the difference with the expiry time.
> -	 */
> -	if (CPUCLOCK_PERTHREAD(timer->it_clock))
> -		now = cpu_clock_sample(clkid, p);
> -	else
> -		now = cpu_clock_sample_group(clkid, p, false);
> +	u64 expires = cpu_timer_getexpires(&timer->it.cpu);
>  
>  	if (now < expires) {
>  		itp->it_value = ns_to_timespec64(expires - now);
> @@ -823,7 +799,28 @@ static void posix_cpu_timer_get(struct k
>  		itp->it_value.tv_nsec = 1;
>  		itp->it_value.tv_sec = 0;
>  	}
> -out:
> +}
> +
> +static void posix_cpu_timer_get(struct k_itimer *timer, struct itimerspec64 *itp)
> +{
> +	clockid_t clkid = CPUCLOCK_WHICH(timer->it_clock);
> +	struct task_struct *p;
> +	u64 now;
> +
> +	rcu_read_lock();
> +	p = cpu_timer_task_rcu(timer);
> +	if (p) {
> +		itp->it_interval = ktime_to_timespec64(timer->it_interval);
> +
> +		if (cpu_timer_getexpires(&timer->it.cpu)) {
> +			if (CPUCLOCK_PERTHREAD(timer->it_clock))
> +				now = cpu_clock_sample(clkid, p);
> +			else
> +				now = cpu_clock_sample_group(clkid, p, false);
> +
> +			__posix_cpu_timer_get(timer, itp, now);
> +		}
> +	}
>  	rcu_read_unlock();
>  }
>