[PATCH v4 3/4] cgroups: subsystem module unloading
Ben Blum
bblum at andrew.cmu.edu
Wed Dec 30 21:14:23 PST 2009
Provides support for unloading modular subsystems.
From: Ben Blum <bblum at 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 parse_cgroupfs_options and rebind_subsystems) (i.e., 0 or 1).
Signed-off-by: Ben Blum <bblum at andrew.cmu.edu>
Acked-by: Li Zefan <lizf at cn.fujitsu.com>
---
Documentation/cgroups/cgroups.txt | 5 +
include/linux/cgroup.h | 4 +
kernel/cgroup.c | 164 +++++++++++++++++++++++++++++++------
3 files changed, 145 insertions(+), 28 deletions(-)
diff --git a/Documentation/cgroups/cgroups.txt b/Documentation/cgroups/cgroups.txt
index 6ffcf81..7527bac 100644
--- a/Documentation/cgroups/cgroups.txt
+++ b/Documentation/cgroups/cgroups.txt
@@ -489,8 +489,9 @@ 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(&its_subsys_struct). It
-should also set its_subsys.module = THIS_MODULE in its .c file.
+module initcall a call to cgroup_load_subsys(), and in its exitcall a
+call to cgroup_unload_subsys(). It should also set its_subsys.module =
+THIS_MODULE in its .c file.
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 9461aed..9be4c22 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -37,6 +37,7 @@ extern void cgroup_exit(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 const 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 d7ca4cf..cc2e1f6 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -878,7 +878,9 @@ void cgroup_release_and_wakeup_rmdir(struct cgroup_subsys_state *css)
}
/*
- * Call with cgroup_mutex held.
+ * Call with cgroup_mutex held. Drops reference counts on modules, including
+ * any duplicate ones that parse_cgroupfs_options took. If this function
+ * returns an error, no reference counts are touched.
*/
static int rebind_subsystems(struct cgroupfs_root *root,
unsigned long final_bits)
@@ -934,6 +936,7 @@ static int rebind_subsystems(struct cgroupfs_root *root,
if (ss->bind)
ss->bind(ss, cgrp);
mutex_unlock(&ss->hierarchy_mutex);
+ /* refcount was already taken, and we're keeping it */
} else if (bit & removed_bits) {
/* We're removing this subsystem */
BUG_ON(ss == NULL);
@@ -947,10 +950,18 @@ static int rebind_subsystems(struct cgroupfs_root *root,
subsys[i]->root = &rootnode;
list_move(&ss->sibling, &rootnode.subsys_list);
mutex_unlock(&ss->hierarchy_mutex);
+ /* 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);
BUG_ON(!cgrp->subsys[i]);
+ /*
+ * a refcount was taken, but we already had one, so
+ * drop the extra reference.
+ */
+ module_put(ss->module);
+ BUG_ON(ss->module && !module_refcount(ss->module));
} else {
/* Subsystem state shouldn't exist */
BUG_ON(cgrp->subsys[i]);
@@ -994,13 +1005,16 @@ struct cgroup_sb_opts {
/*
* Convert a hierarchy specifier into a bitmask of subsystems and flags. Call
- * with cgroup_mutex held to protect the subsys[] array.
+ * with cgroup_mutex held to protect the subsys[] array. This function takes
+ * refcounts on subsystems to be used, unless it returns error, in which case
+ * no refcounts are taken.
*/
-static int parse_cgroupfs_options(char *data,
- struct cgroup_sb_opts *opts)
+static int parse_cgroupfs_options(char *data, struct cgroup_sb_opts *opts)
{
char *token, *o = data ?: "all";
unsigned long mask = (unsigned long)-1;
+ int i;
+ bool module_pin_failed = false;
BUG_ON(!mutex_is_locked(&cgroup_mutex));
@@ -1015,7 +1029,6 @@ static int parse_cgroupfs_options(char *data,
return -EINVAL;
if (!strcmp(token, "all")) {
/* Add all non-disabled subsystems */
- int i;
opts->subsys_bits = 0;
for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
struct cgroup_subsys *ss = subsys[i];
@@ -1038,7 +1051,6 @@ static int parse_cgroupfs_options(char *data,
if (!opts->release_agent)
return -ENOMEM;
} else if (!strncmp(token, "name=", 5)) {
- int i;
const char *name = token + 5;
/* Can't specify an empty name */
if (!strlen(name))
@@ -1062,7 +1074,6 @@ static int parse_cgroupfs_options(char *data,
return -ENOMEM;
} else {
struct cgroup_subsys *ss;
- int i;
for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
ss = subsys[i];
if (ss == NULL)
@@ -1101,9 +1112,54 @@ static int parse_cgroupfs_options(char *data,
if (!opts->subsys_bits && !opts->name)
return -EINVAL;
+ /*
+ * Grab references on all the modules we'll need, so the subsystems
+ * don't dance around before rebind_subsystems attaches them. This may
+ * take duplicate reference counts on a subsystem that's already used,
+ * but rebind_subsystems handles this case.
+ */
+ for (i = CGROUP_BUILTIN_SUBSYS_COUNT; i < CGROUP_SUBSYS_COUNT; i++) {
+ unsigned long bit = 1UL << i;
+
+ if (!(bit & opts->subsys_bits))
+ continue;
+ if (!try_module_get(subsys[i]->module)) {
+ module_pin_failed = true;
+ 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--) {
+ /* drop refcounts only on the ones we took */
+ unsigned long bit = 1UL << i;
+
+ if (!(bit & opts->subsys_bits))
+ continue;
+ module_put(subsys[i]->module);
+ }
+ return -ENOENT;
+ }
+
return 0;
}
+static void drop_parsed_module_refcounts(unsigned long subsys_bits)
+{
+ int i;
+ for (i = CGROUP_BUILTIN_SUBSYS_COUNT; i < CGROUP_SUBSYS_COUNT; i++) {
+ unsigned long bit = 1UL << i;
+
+ if (!(bit & subsys_bits))
+ continue;
+ module_put(subsys[i]->module);
+ }
+}
+
static int cgroup_remount(struct super_block *sb, int *flags, char *data)
{
int ret = 0;
@@ -1120,21 +1176,19 @@ static int cgroup_remount(struct super_block *sb, int *flags, char *data)
if (ret)
goto out_unlock;
- /* Don't allow flags to change at remount */
- if (opts.flags != root->flags) {
- ret = -EINVAL;
- goto out_unlock;
- }
-
- /* Don't allow name to change at remount */
- if (opts.name && strcmp(opts.name, root->name)) {
+ /* Don't allow flags or name to change at remount */
+ if (opts.flags != root->flags ||
+ (opts.name && strcmp(opts.name, root->name))) {
ret = -EINVAL;
+ drop_parsed_module_refcounts(opts.subsys_bits);
goto out_unlock;
}
ret = rebind_subsystems(root, opts.subsys_bits);
- if (ret)
+ if (ret) {
+ drop_parsed_module_refcounts(opts.subsys_bits);
goto out_unlock;
+ }
/* (re)populate subsystem files */
cgroup_populate_dir(cgrp);
@@ -1333,7 +1387,7 @@ static int cgroup_get_sb(struct file_system_type *fs_type,
new_root = cgroup_root_from_opts(&opts);
if (IS_ERR(new_root)) {
ret = PTR_ERR(new_root);
- goto out_err;
+ goto drop_modules;
}
opts.new_root = new_root;
@@ -1342,7 +1396,7 @@ static int cgroup_get_sb(struct file_system_type *fs_type,
if (IS_ERR(sb)) {
ret = PTR_ERR(sb);
cgroup_drop_root(opts.new_root);
- goto out_err;
+ goto drop_modules;
}
root = sb->s_fs_info;
@@ -1398,6 +1452,11 @@ static int cgroup_get_sb(struct file_system_type *fs_type,
free_cg_links(&tmp_cg_links);
goto drop_new_super;
}
+ /*
+ * There must be no failure case after here, since rebinding
+ * takes care of subsystems' refcounts, which are explicitly
+ * dropped in the failure exit path.
+ */
/* EBUSY should be the only error here */
BUG_ON(ret);
@@ -1436,6 +1495,8 @@ static int cgroup_get_sb(struct file_system_type *fs_type,
* any) is not needed
*/
cgroup_drop_root(opts.new_root);
+ /* no subsys rebinding, so refcounts don't change */
+ drop_parsed_module_refcounts(opts.subsys_bits);
}
simple_set_mnt(mnt, sb);
@@ -1445,6 +1506,8 @@ static int cgroup_get_sb(struct file_system_type *fs_type,
drop_new_super:
deactivate_locked_super(sb);
+ drop_modules:
+ drop_parsed_module_refcounts(opts.subsys_bits);
out_err:
kfree(opts.release_agent);
kfree(opts.name);
@@ -3366,13 +3429,6 @@ 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);
return 0;
@@ -3380,6 +3436,64 @@ 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 parse_cgroupfs_options should ensure that it
+ * doesn't start being used while we're killing it off.
+ */
+ BUG_ON(ss->root != &rootnode);
+
+ mutex_lock(&cgroup_mutex);
+ /* deassign the subsys_id */
+ BUG_ON(ss->subsys_id < CGROUP_BUILTIN_SUBSYS_COUNT);
+ subsys[ss->subsys_id] = NULL;
+
+ /* 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);
+}
+EXPORT_SYMBOL_GPL(cgroup_unload_subsys);
+
+/**
* cgroup_init_early - cgroup initialization at system boot
*
* Initialize cgroups at system boot, and initialize any
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