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

[Paul Menage]

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 at 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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