Re: + hugetlb-cgroup-simplify-pre_destroy-callback.patch added to -mm tree

[Michal Hocko <mhocko@suse.cz>]

On Thu 19-07-12 19:18:24, Aneesh Kumar K.V wrote:
> Michal Hocko <mhocko@suse.cz> writes:
> 
> > On Thu 19-07-12 17:51:05, Aneesh Kumar K.V wrote:
> >> Michal Hocko <mhocko@suse.cz> writes:
> >> 
> >> > From 621ed1c9dab63bd82205bd5266eb9974f86a0a3f Mon Sep 17 00:00:00 2001
> >> > From: Michal Hocko <mhocko@suse.cz>
> >> > Date: Thu, 19 Jul 2012 13:23:23 +0200
> >> > Subject: [PATCH] cgroup: keep cgroup_mutex locked for pre_destroy
> >> >
> >> > 3fa59dfb (cgroup: fix potential deadlock in pre_destroy) dropped the
> >> > cgroup_mutex lock while calling pre_destroy callbacks because memory
> >> > controller could deadlock because force_empty triggered reclaim.
> >> > Since "memcg: move charges to root cgroup if use_hierarchy=0" there is
> >> > no reclaim going on from mem_cgroup_force_empty though so we can safely
> >> > keep the cgroup_mutex locked. This has an advantage that no tasks might
> >> > be added during pre_destroy callback and so the handlers don't have to
> >> > consider races when new tasks add new charges. This simplifies the
> >> > implementation.
> >> > ---
> >> >  kernel/cgroup.c |    2 --
> >> >  1 file changed, 2 deletions(-)
> >> >
> >> > diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> >> > index 0f3527d..9dba05d 100644
> >> > --- a/kernel/cgroup.c
> >> > +++ b/kernel/cgroup.c
> >> > @@ -4181,7 +4181,6 @@ again:
> >> >  		mutex_unlock(&cgroup_mutex);
> >> >  		return -EBUSY;
> >> >  	}
> >> > -	mutex_unlock(&cgroup_mutex);
> >> >
> >> >  	/*
> >> >  	 * In general, subsystem has no css->refcnt after pre_destroy(). But
> >> > @@ -4204,7 +4203,6 @@ again:
> >> >  		return ret;
> >> >  	}
> >> >
> >> > -	mutex_lock(&cgroup_mutex);
> >> >  	parent = cgrp->parent;
> >> >  	if (atomic_read(&cgrp->count) || !list_empty(&cgrp->children)) {
> >> >  		clear_bit(CGRP_WAIT_ON_RMDIR, &cgrp->flags);
> >> 
> >> mem_cgroup_force_empty still calls 
> >> 
> >> lru_add_drain_all 
> >>    ->schedule_on_each_cpu
> >>         -> get_online_cpus
> >>            ->mutex_lock(&cpu_hotplug.lock);
> >> 
> >> So wont we deadlock ?
> >
> > Yes you are right. I got it wrong. I thought that the reclaim is the
> > main problem. It won't be that easy then and the origin mm patch
> > (hugetlb-cgroup-simplify-pre_destroy-callback.patch) still needs a fix
> > or to be dropped.
> 
> We just need to remove the VM_BUG_ON() right ? The rest of the patch is
> good right ? Otherwise how about the below

You can keep VM_BUG_ON with the patch below and also remove the check
for cgroup_task_count || &cgroup->children because that is checked in
cgroup_rmdir already.

> NOTE: Do we want to do s/mutex_[un]lock(&cgroup_mutex)/cgroup_[un]lock()/  ?
> 
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index 7981850..01c67f4 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -4151,7 +4151,6 @@ again:
>  		mutex_unlock(&cgroup_mutex);
>  		return -EBUSY;
>  	}
> -	mutex_unlock(&cgroup_mutex);
>  
>  	/*
>  	 * In general, subsystem has no css->refcnt after pre_destroy(). But
> @@ -4171,10 +4170,10 @@ again:
>  	ret = cgroup_call_pre_destroy(cgrp);
>  	if (ret) {
>  		clear_bit(CGRP_WAIT_ON_RMDIR, &cgrp->flags);
> +		mutex_unlock(&cgroup_mutex);
>  		return ret;
>  	}
>  
> -	mutex_lock(&cgroup_mutex);
>  	parent = cgrp->parent;
>  	if (atomic_read(&cgrp->count) || !list_empty(&cgrp->children)) {
>  		clear_bit(CGRP_WAIT_ON_RMDIR, &cgrp->flags);
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index e8ddc00..91c96df 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -4993,9 +4993,18 @@ free_out:
>  
>  static int mem_cgroup_pre_destroy(struct cgroup *cont)
>  {
> +	int ret;
>  	struct mem_cgroup *memcg = mem_cgroup_from_cont(cont);
>  
> -	return mem_cgroup_force_empty(memcg, false);
> +	cgroup_unlock();
> +	/*
> +	 * we call lru_add_drain_all, which end up taking
> +	 * mutex_lock(&cpu_hotplug.lock), But cpuset have
> +	 * the reverse order. So drop the cgroup lock
> +	 */
> +	ret = mem_cgroup_force_empty(memcg, false);
> +	cgroup_unlock();
> +	return ret;
>  }
>  
>  static void mem_cgroup_destroy(struct cgroup *cont)
> 
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-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic

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