linux-kernel - Re: [PATCH] sched/fair: Fix call walk_tg_tree_from() without hold rcu

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Message-ID: <xm26imico4wg.fsf@google.com>
Date:   Mon, 06 Apr 2020 11:17:19 -0700
From:   bsegall@...gle.com
To:     Muchun Song <songmuchun@...edance.com>
Cc:     mingo@...nel.org, peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, linux-kernel@...r.kernel.org,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de
Subject: Re: [PATCH] sched/fair: Fix call walk_tg_tree_from() without hold rcu_lock

Muchun Song <songmuchun@...edance.com> writes:

> The walk_tg_tree_from() caller must hold rcu_lock, but the caller
> do not call rcu_read_lock() in the unthrottle_cfs_rq(). The
> unthrottle_cfs_rq() is used in 3 places. There are
> distribute_cfs_runtime(), unthrottle_offline_cfs_rqs() and
> tg_set_cfs_bandwidth(). The former 2 already hold the rcu lock,
> but the last one does not. So fix it with calling rcu_read_lock()
> in the unthrottle_cfs_rq().

It might be a tiny bit better to put it in the tg_set_cfs_bandwidth
instead, but the other two sources were kinda by accident, so this is
reasonable too.

Reviewed-by: Ben Segall <bsegall@...gle.com>

>
> Signed-off-by: Muchun Song <songmuchun@...edance.com>
> ---
>  kernel/sched/fair.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 6f05843c76d7d..870853c47b63c 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4782,7 +4782,9 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq)
>  	raw_spin_unlock(&cfs_b->lock);
>  
>  	/* update hierarchical throttle state */
> +	rcu_read_lock();
>  	walk_tg_tree_from(cfs_rq->tg, tg_nop, tg_unthrottle_up, (void *)rq);
> +	rcu_read_unlock();
>  
>  	if (!cfs_rq->load.weight)
>  		return;