linux-kernel - Re: [PATCH v6 3/5] cgroup/cpuset: make callback

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Message-ID: <20190204120236.GC17550@hirez.programming.kicks-ass.net>
Date:   Mon, 4 Feb 2019 13:02:36 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Juri Lelli <juri.lelli@...hat.com>
Cc:     mingo@...hat.com, rostedt@...dmis.org, tj@...nel.org,
        linux-kernel@...r.kernel.org, luca.abeni@...tannapisa.it,
        claudio@...dence.eu.com, tommaso.cucinotta@...tannapisa.it,
        bristot@...hat.com, mathieu.poirier@...aro.org, lizefan@...wei.com,
        cgroups@...r.kernel.org
Subject: Re: [PATCH v6 3/5] cgroup/cpuset: make callback_lock raw

On Thu, Jan 17, 2019 at 09:47:37AM +0100, Juri Lelli wrote:
> @@ -3233,11 +3233,11 @@ void cpuset_cpus_allowed(struct task_struct *tsk, struct cpumask *pmask)
>  {
>  	unsigned long flags;
>  
> -	spin_lock_irqsave(&callback_lock, flags);
> +	raw_spin_lock_irqsave(&callback_lock, flags);
>  	rcu_read_lock();
>  	guarantee_online_cpus(task_cs(tsk), pmask);
>  	rcu_read_unlock();
> -	spin_unlock_irqrestore(&callback_lock, flags);
> +	raw_spin_unlock_irqrestore(&callback_lock, flags);
>  }
>  
>  void cpuset_cpus_allowed_fallback(struct task_struct *tsk)
> @@ -3285,11 +3285,11 @@ nodemask_t cpuset_mems_allowed(struct task_struct *tsk)
>  	nodemask_t mask;
>  	unsigned long flags;
>  
> -	spin_lock_irqsave(&callback_lock, flags);
> +	raw_spin_lock_irqsave(&callback_lock, flags);
>  	rcu_read_lock();
>  	guarantee_online_mems(task_cs(tsk), &mask);
>  	rcu_read_unlock();
> -	spin_unlock_irqrestore(&callback_lock, flags);
> +	raw_spin_unlock_irqrestore(&callback_lock, flags);
>  
>  	return mask;
>  }
> @@ -3381,14 +3381,14 @@ bool __cpuset_node_allowed(int node, gfp_t gfp_mask)
>  		return true;
>  
>  	/* Not hardwall and node outside mems_allowed: scan up cpusets */
> -	spin_lock_irqsave(&callback_lock, flags);
> +	raw_spin_lock_irqsave(&callback_lock, flags);
>  
>  	rcu_read_lock();
>  	cs = nearest_hardwall_ancestor(task_cs(current));
>  	allowed = node_isset(node, cs->mems_allowed);
>  	rcu_read_unlock();
>  
> -	spin_unlock_irqrestore(&callback_lock, flags);
> +	raw_spin_unlock_irqrestore(&callback_lock, flags);
>  	return allowed;
>  }

These three appear to be a user-controlled O(n) (depth of cgroup tree).
Which is basically bad for raw_spinlock_t.

The Changelog should really have mentioned this; and ideally we'd
somehow avoid this.