Re: [PATCH v2 4/5] don't take cgroup_mutex in destroy()

On 04/23/2012 11:31 PM, KAMEZAWA Hiroyuki wrote:

(2012/04/24 4:37), Glauber Costa wrote:

quoted

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<redacted>
CC: Tejun Heo<redacted>
CC: Li Zefan<redacted>
CC: Kamezawa Hiroyuki<redacted>
CC: Vivek Goyal<redacted>
---
  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?