[PATCH] Move cgroups destroy() callbacks to cgroup_diput()

[Paul Menage <menage@google.com>]

Move cgroups destroy() callbacks to cgroup_diput() 

Move the calls to the cgroup subsystem destroy() methods from
cgroup_rmdir() to cgroup_diput(). This allows control file reads and
writes to access their subsystem state without having to be concerned
with locking against cgroup destruction - the control file dentry will
keep the cgroup and its subsystem state objects alive until the file
is closed.

The documentation is updated to reflect the changed semantics of
destroy(); additionally the locking comments for destroy() and some
other methods were clarified and decrustified.

Signed-off-by: Paul Menage <menage@google.com>

---
Documentation/cgroups.txt |   22 +++++++++++-----------
kernel/cgroup.c           |   36 ++++++++++++++++++++++++------------
2 files changed, 35 insertions(+), 23 deletions(-)

Index: container-2.6.23-mm1/kernel/cgroup.c
===================================================================
--- container-2.6.23-mm1.orig/kernel/cgroup.c
+++ container-2.6.23-mm1/kernel/cgroup.c
@@ -592,6 +592,7 @@ static void cgroup_diput(struct dentry *
	/* is dentry a directory ? if so, kfree() associated cgroup */
	if (S_ISDIR(inode->i_mode)) {
		struct cgroup *cgrp = dentry->d_fsdata;
+		struct cgroup_subsys *ss;
		BUG_ON(!(cgroup_is_removed(cgrp)));
		/* It's possible for external users to be holding css
		 * reference counts on a cgroup; css_put() needs to
@@ -600,6 +601,23 @@ static void cgroup_diput(struct dentry *
		 * queue the cgroup to be handled by the release
		 * agent */
		synchronize_rcu();
+
+		mutex_lock(&cgroup_mutex);
+		/*
+		 * Release the subsystem state objects.
+		 */
+		for_each_subsys(cgrp->root, ss) {
+			if (cgrp->subsys[ss->subsys_id])
+				ss->destroy(ss, cgrp);
+		}
+
+		cgrp->root->number_of_cgroups--;
+		mutex_unlock(&cgroup_mutex);
+
+		/* Drop the active superblock reference that we took when we
+		 * created the cgroup */
+		deactivate_super(cgrp->root->sb);
+
		kfree(cgrp);
	}
	iput(inode);
@@ -1333,6 +1351,10 @@ static ssize_t cgroup_common_file_write(

	mutex_lock(&cgroup_mutex);

+	/*
+	 * This was already checked for in cgroup_file_write(), but
+	 * check again now we're holding cgroup_mutex.
+	 */
	if (cgroup_is_removed(cgrp)) {
		retval = -ENODEV;
		goto out2;
@@ -1388,7 +1410,7 @@ static ssize_t cgroup_file_write(struct 
	struct cftype *cft = __d_cft(file->f_dentry);
	struct cgroup *cgrp = __d_cgrp(file->f_dentry->d_parent);

-	if (!cft)
+	if (!cft || cgroup_is_removed(cgrp))
		return -ENODEV;
	if (cft->write)
		return cft->write(cgrp, cft, file, buf, nbytes, ppos);
@@ -1458,7 +1480,7 @@ static ssize_t cgroup_file_read(struct f
	struct cftype *cft = __d_cft(file->f_dentry);
	struct cgroup *cgrp = __d_cgrp(file->f_dentry->d_parent);

-	if (!cft)
+	if (!cft || cgroup_is_removed(cgrp))
		return -ENODEV;

	if (cft->read)
@@ -2139,7 +2161,6 @@ static int cgroup_rmdir(struct inode *un
	struct cgroup *cgrp = dentry->d_fsdata;
	struct dentry *d;
	struct cgroup *parent;
-	struct cgroup_subsys *ss;
	struct super_block *sb;
	struct cgroupfs_root *root;

@@ -2164,11 +2185,6 @@ static int cgroup_rmdir(struct inode *un
		return -EBUSY;
	}

-	for_each_subsys(root, ss) {
-		if (cgrp->subsys[ss->subsys_id])
-			ss->destroy(ss, cgrp);
-	}
-
	spin_lock(&release_list_lock);
	set_bit(CGRP_REMOVED, &cgrp->flags);
	if (!list_empty(&cgrp->release_list))
@@ -2183,15 +2199,11 @@ static int cgroup_rmdir(struct inode *un

	cgroup_d_remove_dir(d);
	dput(d);
-	root->number_of_cgroups--;

	set_bit(CGRP_RELEASABLE, &parent->flags);
	check_for_release(parent);

	mutex_unlock(&cgroup_mutex);
-	/* Drop the active superblock reference that we took when we
-	 * created the cgroup */
-	deactivate_super(sb);
	return 0;
}

Index: container-2.6.23-mm1/Documentation/cgroups.txt
===================================================================
--- container-2.6.23-mm1.orig/Documentation/cgroups.txt
+++ container-2.6.23-mm1/Documentation/cgroups.txt
@@ -456,7 +456,7 @@ methods are create/destroy. Any others t
be successful no-ops.

struct cgroup_subsys_state *create(struct cgroup *cont)
-LL=cgroup_mutex
+(cgroup_mutex held by caller)

Called to create a subsystem state object for a cgroup. The
subsystem should allocate its subsystem state object for the passed
@@ -471,14 +471,19 @@ it's the root of the hierarchy) and may 
initialization code.

void destroy(struct cgroup *cont)
-LL=cgroup_mutex
+(cgroup_mutex held by caller)

-The cgroup system is about to destroy the passed cgroup; the
-subsystem should do any necessary cleanup
+The cgroup system is about to destroy the passed cgroup; the subsystem
+should do any necessary cleanup and free its subsystem state
+object. By the time this method is called, the cgroup has already been
+unlinked from the file system and from the child list of its parent;
+cgroup->parent is still valid. (Note - can also be called for a
+newly-created cgroup if an error occurs after this subsystem's
+create() method has been called for the new cgroup).

int can_attach(struct cgroup_subsys *ss, struct cgroup *cont,
	       struct task_struct *task)
-LL=cgroup_mutex
+(cgroup_mutex held by caller)

Called prior to moving a task into a cgroup; if the subsystem
returns an error, this will abort the attach operation.  If a NULL
@@ -489,25 +494,20 @@ remain valid while the caller holds cgro

void attach(struct cgroup_subsys *ss, struct cgroup *cont,
	    struct cgroup *old_cont, struct task_struct *task)
-LL=cgroup_mutex
-

Called after the task has been attached to the cgroup, to allow any
post-attachment activity that requires memory allocations or blocking.

void fork(struct cgroup_subsy *ss, struct task_struct *task)
-LL=callback_mutex, maybe read_lock(tasklist_lock)

Called when a task is forked into a cgroup. Also called during
registration for all existing tasks.

void exit(struct cgroup_subsys *ss, struct task_struct *task)
-LL=callback_mutex

Called during task exit

int populate(struct cgroup_subsys *ss, struct cgroup *cont)
-LL=none

Called after creation of a cgroup to allow a subsystem to populate
the cgroup directory with file entries.  The subsystem should make
@@ -524,7 +524,7 @@ example in cpusets, no task may attach b
up.

void bind(struct cgroup_subsys *ss, struct cgroup *root)
-LL=callback_mutex
+(cgroup_mutex held by caller)

Called when a cgroup subsystem is rebound to a different hierarchy
and root cgroup. Currently this will only involve movement between
-
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