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

---
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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