| Re: [PATCH v2 4/5] don't take cgroup_mutex in destroy() [message #46074 is a reply to message #46048] |
Wed, 25 April 2012 08:01   |
Li Zefan
Messages: 90
Registered: February 2008
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Member |
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Glauber Costa wrote:
> On 04/23/2012 11:31 PM, KAMEZAWA Hiroyuki wrote:
>> (2012/04/24 4:37), Glauber Costa wrote:
>>
>>> Most of the destroy functions are only doing very simple things
>>> like freeing memory.
>>>
>>> The ones who goes through lists and such, already use its own
>>> locking for those.
>>>
>>> * The cgroup itself won't go away until we free it, (after destroy)
>>> * The parent won't go away because we hold a reference count
>>> * There are no more tasks in the cgroup, and the cgroup is declared
>>> dead (cgroup_is_removed() == true)
>>>
>>> [v2: don't cgroup_lock the freezer and blkcg ]
>>>
>>> Signed-off-by: Glauber Costa<glommer@parallels.com>
>>> CC: Tejun Heo<tj@kernel.org>
>>> CC: Li Zefan<lizefan@huawei.com>
>>> CC: Kamezawa Hiroyuki<kamezawa.hiroyu@jp.fujitsu.com>
>>> CC: Vivek Goyal<vgoyal@redhat.com>
>>> ---
>>> kernel/cgroup.c | 9 ++++-----
>>> 1 files changed, 4 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
>>> index 932c318..976d332 100644
>>> --- a/kernel/cgroup.c
>>> +++ b/kernel/cgroup.c
>>> @@ -869,13 +869,13 @@ static void cgroup_diput(struct dentry *dentry, struct inode *inode)
>>> * agent */
>>> synchronize_rcu();
>>>
>>> - mutex_lock(&cgroup_mutex);
>>> /*
>>> * Release the subsystem state objects.
>>> */
>>> for_each_subsys(cgrp->root, ss)
>>> ss->destroy(cgrp);
>>>
>>> + mutex_lock(&cgroup_mutex);
>>> cgrp->root->number_of_cgroups--;
>>> mutex_unlock(&cgroup_mutex);
>>>
>>> @@ -3994,13 +3994,12 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
>>>
>>> err_destroy:
>>>
>>> + mutex_unlock(&cgroup_mutex);
>>> for_each_subsys(root, ss) {
>>> if (cgrp->subsys[ss->subsys_id])
>>> ss->destroy(cgrp);
>>> }
>>>
>>> - mutex_unlock(&cgroup_mutex);
>>> -
>>> /* Release the reference count that we took on the superblock */
>>> deactivate_super(sb);
>>>
>>> @@ -4349,9 +4348,9 @@ int __init_or_module cgroup_load_subsys(struct cgroup_subsys *ss)
>>> int ret = cgroup_init_idr(ss, css);
>>> if (ret) {
>>> dummytop->subsys[ss->subsys_id] = NULL;
>>> + mutex_unlock(&cgroup_mutex);
>>> ss->destroy(dummytop);
>>> subsys[i] = NULL;
>>> - mutex_unlock(&cgroup_mutex);
>>> return ret;
>>> }
>>> }
>>> @@ -4447,10 +4446,10 @@ void cgroup_unload_subsys(struct cgroup_subsys *ss)
>>> * pointer to find their state. note that this also takes care of
>>> * freeing the css_id.
>>> */
>>> + mutex_unlock(&cgroup_mutex);
>>> ss->destroy(dummytop);
>>> dummytop->subsys[ss->subsys_id] = NULL;
>>>
>>
>> I'm not fully sure but...dummytop->subsys[] update can be done without locking ?
>>
> I don't see a reason why updates to subsys[] after destruction shouldn't
> be safe. But maybe I am wrong.
>
> Tejun? Li?
>
It's safe for dummpytop->subsys[], but it makes the code a bit subtle.
The worst part is, it's not safe to NULLify subsys[i] without cgroup_mutex. It should be
ok to do that before calling ->destroy(), but again the code becomes a bit subtler.
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