Michal Hocko [off-list ref] writes:

On Thu 19-07-12 17:51:05, Aneesh Kumar K.V wrote:

quoted

Michal Hocko [off-list ref] writes:

quoted

From 621ed1c9dab63bd82205bd5266eb9974f86a0a3f Mon Sep 17 00:00:00 2001
From: Michal Hocko <redacted>
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)