(2012/07/20 10:05), Kamezawa Hiroyuki wrote:

(2012/07/19 22:48), Aneesh Kumar K.V wrote:

quoted

Michal Hocko [off-list ref] writes:

quoted

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.

Aha, then the problematic schedule_on_each_cpu(), Andrew pointed out in this month,
is in front of us :( ...and drain_all_stock_sync() should be fixed too.
Hmm...

quoted

We just need to remove the VM_BUG_ON() right ? The rest of the patch is
good right ? Otherwise how about the below

I'm personally okay but....ugly ?

Hmm, can't cgroup_lock() be implemented as


void cgroup_lock()
{
	get_online_cpus()
	lock_memory_hotplug()
	mutex_lock(&cgroup_mutex);
}


?

Thanks,
-Kame