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Date: Wed, 21 May 2014 11:38:35 +0400
From: Roman Gushchin <klamm@...dex-team.ru>
To: Ben Segall <bsegall@...gle.com>
Cc: Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...hat.com>,
Roman Gushchin <klamm@...dex-team.ru>, pjt@...gle.com,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched: fix unlocked reads of some cfs_b->quota/period
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@...dex-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@...gle.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);
> }
>
--
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