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

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Message-ID: <20190204120711.GA17582@hirez.programming.kicks-ass.net>
Date:   Mon, 4 Feb 2019 13:07:11 +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 Mon, Feb 04, 2019 at 01:02:36PM +0100, Peter Zijlstra wrote:
> 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.

N/m avoiding it; we have this all over the place, just mention it..