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

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

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

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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