linux-kernel - Re: [patch V2 28/38] posix-cpu-timers: Restructure expiry array

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Message-ID: <20190826163204.GA14309@lenoir>
Date:   Mon, 26 Aug 2019 18:32:05 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        John Stultz <john.stultz@...aro.org>,
        Anna-Maria Behnsen <anna-maria@...utronix.de>,
        Christoph Hellwig <hch@....de>
Subject: Re: [patch V2 28/38] posix-cpu-timers: Restructure expiry array

On Wed, Aug 21, 2019 at 09:09:15PM +0200, Thomas Gleixner wrote:
>  /**
> - * task_cputimers_expired - Compare two task_cputime entities.
> + * task_cputimers_expired - Check whether posix CPU timers are expired
>   *
>   * @samples:	Array of current samples for the CPUCLOCK clocks
> - * @expiries:	Array of expiry values for the CPUCLOCK clocks
> + * @pct:	Pointer to a posix_cputimers container
>   *
> - * Returns true if any mmember of @samples is greater than the corresponding
> - * member of @expiries if that member is non zero. False otherwise
> + * Returns true if any member of @samples is greater than the corresponding
> + * member of @pct->bases[CLK].nextevt. False otherwise
>   */
> -static inline bool task_cputimers_expired(const u64 *sample, const u64 *expiries)
> +static inline bool
> +task_cputimers_expired(const u64 *sample, struct posix_cputimers *pct)
>  {
>  	int i;
>  
>  	for (i = 0; i < CPUCLOCK_MAX; i++) {
> -		if (expiries[i] && sample[i] >= expiries[i])
> +		if (sample[i] >= pct->bases[i].nextevt)

You may have false positive here if you don't check if pct->bases[i].nextevt
is 0. Probably no big deal by the end of the series since you change that 0
for KTIME_MAX later but right now it might hurt bisection with performance
issues (locking sighand at every tick...).

[...]

> @@ -1176,7 +1182,7 @@ void run_posix_cpu_timers(void)
>  void set_process_cpu_timer(struct task_struct *tsk, unsigned int clkid,
>  			   u64 *newval, u64 *oldval)
>  {
> -	u64 now, *expiry = tsk->signal->posix_cputimers.expiries + clkid;
> +	u64 now, *nextevt = &tsk->signal->posix_cputimers.bases[clkid].nextevt;

You're dereferencing the pointer before checking clkid sanity below.

>  
>  	if (WARN_ON_ONCE(clkid >= CPUCLOCK_SCHED))
>  		return;
> @@ -1207,8 +1213,8 @@ void set_process_cpu_timer(struct task_s
>  	 * Update expiration cache if this is the earliest timer. CPUCLOCK_PROF
>  	 * expiry cache is also used by RLIMIT_CPU!.
>  	 */
> -	if (expires_gt(*expiry, *newval))
> -		*expiry = *newval;
> +	if (expires_gt(*nextevt, *newval))
> +		*nextevt = *newval;
>  
>  	tick_dep_set_signal(tsk->signal, TICK_DEP_BIT_POSIX_TIMER);
>  }
> 
>