Re: [PATCH] sched: fix unlocked reads of some cfs_b->quota/period

[Roman Gushchin <klamm@yandex-team.ru>]

I tested this patch together with my previous patch 
("sched: tg_set_cfs_bandwidth() causes rq->lock deadlock"),
and can confirm, that all lockups are gone away.

Tested-by: Roman Gushchin <klamm@yandex-team.ru>

At Mon, 19 May 2014 15:49:45 -0700,
Ben Segall wrote:
> 
> sched_cfs_period_timer read cfs_b->period without locks before calling
> do_sched_cfs_period_timer, and similarly unthrottle_offline_cfs_rqs
> would read cfs_b->period without the right lock. Thus a simultaneous
> change of bandwidth could cause corruption on any platform where ktime_t
> or u64 writes/reads are not atomic.
> 
> Extend cfs_b->lock from do_sched_cfs_period_timer to include the read of
> cfs_b->period to solve that issue; unthrottle_offline_cfs_rqs can just
> use 1 rather than the exact quota, much like distribute_cfs_runtime
> does.
> 
> There is also an unlocked read of cfs_b->runtime_expires, but a race
> there would only delay runtime expiry by a tick. Still, the comparison
> should just be != anyway, which clarifies even that problem.
> 
> Signed-off-by: Ben Segall <bsegall@google.com>
> ---
>  kernel/sched/fair.c |   38 +++++++++++++++++++++-----------------
>  1 file changed, 21 insertions(+), 17 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index c9617b7..60eddde 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3224,10 +3224,12 @@ static void expire_cfs_rq_runtime(struct cfs_rq *cfs_rq)
>  	 * has not truly expired.
>  	 *
>  	 * Fortunately we can check determine whether this the case by checking
> -	 * whether the global deadline has advanced.
> +	 * whether the global deadline has advanced. It is valid to compare
> +	 * cfs_b->runtime_expires without any locks since we only care about
> +	 * exact equality, so a partial write will still work.
>  	 */
>  
> -	if ((s64)(cfs_rq->runtime_expires - cfs_b->runtime_expires) >= 0) {
> +	if (cfs_rq->runtime_expires != cfs_b->runtime_expires) {
>  		/* extend local deadline, drift is bounded above by 2 ticks */
>  		cfs_rq->runtime_expires += TICK_NSEC;
>  	} else {
> @@ -3456,21 +3458,21 @@ next:
>  static int do_sched_cfs_period_timer(struct cfs_bandwidth *cfs_b, int overrun)
>  {
>  	u64 runtime, runtime_expires;
> -	int idle = 1, throttled;
> +	int throttled;
>  
> -	raw_spin_lock(&cfs_b->lock);
>  	/* no need to continue the timer with no bandwidth constraint */
>  	if (cfs_b->quota == RUNTIME_INF)
> -		goto out_unlock;
> +		goto out_deactivate;
>  
>  	throttled = !list_empty(&cfs_b->throttled_cfs_rq);
> -	/* idle depends on !throttled (for the case of a large deficit) */
> -	idle = cfs_b->idle && !throttled;
>  	cfs_b->nr_periods += overrun;
>  
> -	/* if we're going inactive then everything else can be deferred */
> -	if (idle)
> -		goto out_unlock;
> +	/*
> +	 * idle depends on !throttled (for the case of a large deficit), and if
> +	 * we're going inactive then everything else can be deferred
> +	 */
> +	if (cfs_b->idle && !throttled)
> +		goto out_deactivate;
>  
>  	/*
>  	 * if we have relooped after returning idle once, we need to update our
> @@ -3484,7 +3486,7 @@ static int do_sched_cfs_period_timer(struct cfs_bandwidth *cfs_b, int overrun)
>  	if (!throttled) {
>  		/* mark as potentially idle for the upcoming period */
>  		cfs_b->idle = 1;
> -		goto out_unlock;
> +		return 0;
>  	}
>  
>  	/* account preceding periods in which throttling occurred */
> @@ -3524,12 +3526,12 @@ static int do_sched_cfs_period_timer(struct cfs_bandwidth *cfs_b, int overrun)
>  	 * timer to remain active while there are any throttled entities.)
>  	 */
>  	cfs_b->idle = 0;
> -out_unlock:
> -	if (idle)
> -		cfs_b->timer_active = 0;
> -	raw_spin_unlock(&cfs_b->lock);
>  
> -	return idle;
> +	return 0;
> +
> +out_deactivate:
> +	cfs_b->timer_active = 0;
> +	return 1;
>  }
>  
>  /* a cfs_rq won't donate quota below this amount */
> @@ -3706,6 +3708,7 @@ static enum hrtimer_restart sched_cfs_period_timer(struct hrtimer *timer)
>  	int overrun;
>  	int idle = 0;
>  
> +	raw_spin_lock(&cfs_b->lock);
>  	for (;;) {
>  		now = hrtimer_cb_get_time(timer);
>  		overrun = hrtimer_forward(timer, now, cfs_b->period);
> @@ -3715,6 +3718,7 @@ static enum hrtimer_restart sched_cfs_period_timer(struct hrtimer *timer)
>  
>  		idle = do_sched_cfs_period_timer(cfs_b, overrun);
>  	}
> +	raw_spin_unlock(&cfs_b->lock);
>  
>  	return idle ? HRTIMER_NORESTART : HRTIMER_RESTART;
>  }
> @@ -3783,7 +3787,7 @@ static void __maybe_unused unthrottle_offline_cfs_rqs(struct rq *rq)
>  		 * clock_task is not advancing so we just need to make sure
>  		 * there's some valid quota amount
>  		 */
> -		cfs_rq->runtime_remaining = cfs_b->quota;
> +		cfs_rq->runtime_remaining = 1;
>  		if (cfs_rq_throttled(cfs_rq))
>  			unthrottle_cfs_rq(cfs_rq);
>  	}
>