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Subject[RFC] [PATCH 4/5] cgroups: subsystem module unloading
Provides support for unloading modular subsystems.

From: Ben Blum <bblum@andrew.cmu.edu>

This patch adds a new function cgroup_unload_subsys which is to be used for
removing a loaded subsystem during module deletion. Reference counting of the
subsystems' modules is moved from once (at load time) to once per attached
hierarchy (in rebind_subsystems) (i.e., 0 or 1).

It also adds a proper module_delete call in net/sched/cls_cgroup.c.

Signed-off-by: Ben Blum <bblum@andrew.cmu.edu>
---

Documentation/cgroups/cgroups.txt | 3 +
include/linux/cgroup.h | 4 +-
kernel/cgroup.c | 92 ++++++++++++++++++++++++++++++++-----
net/sched/cls_cgroup.c | 3 -
4 files changed, 86 insertions(+), 16 deletions(-)


diff --git a/Documentation/cgroups/cgroups.txt b/Documentation/cgroups/cgroups.txt
index dd0d6f1..110228e 100644
--- a/Documentation/cgroups/cgroups.txt
+++ b/Documentation/cgroups/cgroups.txt
@@ -493,7 +493,8 @@ Each subsystem should:
- define a cgroup_subsys object called <name>_subsys

If a subsystem can be compiled as a module, it should also have in its
-module initcall a call to cgroup_load_subsys().
+module initcall a call to cgroup_load_subsys(), and in its exitcall a
+call to cgroup_unload_subsys().

Each subsystem may export the following methods. The only mandatory
methods are create/destroy. Any others that are null are presumed to
diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index c8474c4..1cbb07f 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -39,6 +39,7 @@ extern void cgroup_fork_failed(struct task_struct *p, int run_callbacks,
extern int cgroupstats_build(struct cgroupstats *stats,
struct dentry *dentry);
extern int cgroup_load_subsys(struct cgroup_subsys *ss);
+extern void cgroup_unload_subsys(struct cgroup_subsys *ss);

extern struct file_operations proc_cgroup_operations;

@@ -264,7 +265,8 @@ struct css_set {
/*
* Set of subsystem states, one for each subsystem. This array
* is immutable after creation apart from the init_css_set
- * during subsystem registration (at boot time).
+ * during subsystem registration (at boot time) and modular subsystem
+ * loading/unloading.
*/
struct cgroup_subsys_state *subsys[CGROUP_SUBSYS_COUNT];

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 858a786..c2c7681 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -883,7 +883,7 @@ static int rebind_subsystems(struct cgroupfs_root *root,
{
unsigned long added_bits, removed_bits;
struct cgroup *cgrp = &root->top_cgroup;
- int i;
+ int i, module_pin_failed = 0;

BUG_ON(!rwsem_is_locked(&subsys_mutex));

@@ -914,6 +914,27 @@ static int rebind_subsystems(struct cgroupfs_root *root,
if (root->number_of_cgroups > 1)
return -EBUSY;

+ /* pin the modules for all subsystems that will stop being free. we
+ * don't drop refcounts on removed subsystems until later, since there
+ * are failure cases here. */
+ for (i = CGROUP_BUILTIN_SUBSYS_COUNT; i < CGROUP_SUBSYS_COUNT; i++) {
+ unsigned long bit = 1UL << i;
+ if (!(bit & added_bits))
+ continue;
+ if (!try_module_get(subsys[i]->module)) {
+ module_pin_failed = 1;
+ break;
+ }
+ }
+ if (module_pin_failed) {
+ /* oops, one of the modules was going away. this means that we
+ * raced with a module_delete call, and to the user this is
+ * essentially a "subsystem doesn't exist" case. */
+ for (i--; i >= CGROUP_BUILTIN_SUBSYS_COUNT; i--)
+ module_put(subsys[i]->module);
+ return -ENOENT;
+ }
+
/* Process each subsystem */
for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
struct cgroup_subsys *ss = subsys[i];
@@ -932,8 +953,7 @@ static int rebind_subsystems(struct cgroupfs_root *root,
if (ss->bind)
ss->bind(ss, cgrp);
mutex_unlock(&ss->hierarchy_mutex);
- /* TODO: If subsystem unloading support, need to take
- * a reference on the subsystem here. */
+ /* we already got the reference for this subsystem. */
} else if (bit & removed_bits) {
/* We're removing this subsystem */
BUG_ON(ss == NULL);
@@ -947,8 +967,8 @@ static int rebind_subsystems(struct cgroupfs_root *root,
subsys[i]->root = &rootnode;
list_move(&ss->sibling, &rootnode.subsys_list);
mutex_unlock(&ss->hierarchy_mutex);
- /* TODO: If subsystem unloading support, drop refcount
- * here. */
+ /* subsystem is now free - drop reference on module */
+ module_put(ss->module);
} else if (bit & final_bits) {
/* Subsystem state should already exist */
BUG_ON(ss == NULL);
@@ -1395,7 +1415,7 @@ static int cgroup_get_sb(struct file_system_type *fs_type,
}

ret = rebind_subsystems(root, root->subsys_bits);
- if (ret == -EBUSY) {
+ if (ret == -EBUSY || ret == -ENOENT) {
mutex_unlock(&cgroup_mutex);
mutex_unlock(&inode->i_mutex);
free_cg_links(&tmp_cg_links);
@@ -1404,7 +1424,7 @@ static int cgroup_get_sb(struct file_system_type *fs_type,
/* done with subsys stuff and no other failure case */
up_read(&subsys_mutex);

- /* EBUSY should be the only error here */
+ /* EBUSY and ENOENT should be the only errors here */
BUG_ON(ret);

list_add(&root->root_list, &roots);
@@ -3760,11 +3780,6 @@ int __init_or_module cgroup_load_subsys(struct cgroup_subsys *ss)
lockdep_set_class(&ss->hierarchy_mutex, &ss->subsys_key);
ss->active = 1;

- /* pin the subsystem's module so it doesn't go away. this shouldn't
- * fail, since the module's initcall calls us.
- * TODO: with module unloading, move this elsewhere */
- BUG_ON(!try_module_get(ss->module));
-
/* success! */
mutex_unlock(&cgroup_mutex);
up_write(&subsys_mutex);
@@ -3773,6 +3788,59 @@ int __init_or_module cgroup_load_subsys(struct cgroup_subsys *ss)
EXPORT_SYMBOL_GPL(cgroup_load_subsys);

/**
+ * cgroup_unload_subsys: unload a modular subsystem
+ * @ss: the subsystem to unload
+ *
+ * This function should be called in a modular subsystem's exitcall. When this
+ * function is invoked, the refcount on the subsystem's module will be 0, so
+ * the subsystem will not be attached to any hierarchy.
+ */
+void cgroup_unload_subsys(struct cgroup_subsys *ss)
+{
+ struct cg_cgroup_link *link;
+ struct hlist_head *hhead;
+
+ BUG_ON(ss->module == NULL);
+
+ /* we shouldn't be called if the subsystem is in use, and the use of
+ * try_module_get in rebind_subsystems should ensure that it doesn't
+ * start being used while we're killing it off. */
+ BUG_ON(ss->root != &rootnode);
+
+ down_write(&subsys_mutex);
+ /* deassign the subsys_id */
+ BUG_ON(ss->subsys_id < CGROUP_BUILTIN_SUBSYS_COUNT);
+ subsys[ss->subsys_id] = NULL;
+
+ mutex_lock(&cgroup_mutex);
+ /* remove subsystem from rootnode's list of subsystems */
+ list_del(&ss->sibling);
+
+ /* disentangle the css from all css_sets attached to the dummytop. as
+ * in loading, we need to pay our respects to the hashtable gods. */
+ write_lock(&css_set_lock);
+ list_for_each_entry(link, &dummytop->css_sets, cgrp_link_list) {
+ struct css_set *cg = link->cg;
+ hlist_del(&cg->hlist);
+ BUG_ON(!cg->subsys[ss->subsys_id]);
+ cg->subsys[ss->subsys_id] = NULL;
+ hhead = css_set_hash(cg->subsys);
+ hlist_add_head(&cg->hlist, hhead);
+ }
+ write_unlock(&css_set_lock);
+
+ /* remove subsystem's css from the dummytop and free it - need to free
+ * before marking as null because ss->destroy needs the cgrp->subsys
+ * pointer to find their state. */
+ ss->destroy(ss, dummytop);
+ dummytop->subsys[ss->subsys_id] = NULL;
+
+ mutex_unlock(&cgroup_mutex);
+ up_write(&subsys_mutex);
+}
+EXPORT_SYMBOL_GPL(cgroup_unload_subsys);
+
+/**
* cgroup_init_early - cgroup initialization at system boot
*
* Initialize cgroups at system boot, and initialize any
diff --git a/net/sched/cls_cgroup.c b/net/sched/cls_cgroup.c
index df9723b..7f27d2c 100644
--- a/net/sched/cls_cgroup.c
+++ b/net/sched/cls_cgroup.c
@@ -300,8 +300,7 @@ static int __init init_cgroup_cls(void)
static void __exit exit_cgroup_cls(void)
{
unregister_tcf_proto_ops(&cls_cgroup_ops);
- /* TODO: unload subsystem. for now, the try_module_get in load_subsys
- * prevents us from getting here. */
+ cgroup_unload_subsys(&net_cls_subsys);
}

module_init(init_cgroup_cls);

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