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cgroup: add cgroup_root_mutex 

cgroup wants to make threadgroup stable while modifying cgroup
hierarchies which will introduce locking dependency on
cred_guard_mutex from cgroup_mutex.  This unfortunately completes
circular dependency.

 A. cgroup_mutex -> cred_guard_mutex -> s_type->i_mutex_key -> namespace_sem
 B. namespace_sem -> cgroup_mutex

B is from cgroup_show_options() and this patch breaks it by
introducing another mutex cgroup_root_mutex which nests inside
cgroup_mutex and protects cgroupfs_root.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Li Zefan <lizf@cn.fujitsu.com>
Cc: Oleg Nesterov <oleg@redhat.com>
This commit is contained in:
Tejun Heo 2011-12-12 18:12:21 -08:00
parent 467de1fc67
commit e25e2cbb4c
1 changed files with 42 additions and 22 deletions
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64
kernel/cgroup.c
View File

@ -63,7 +63,24 @@
#include <linux/atomic.h>
/*
* cgroup_mutex is the master lock. Any modification to cgroup or its
* hierarchy must be performed while holding it.
*
* cgroup_root_mutex nests inside cgroup_mutex and should be held to modify
* cgroupfs_root of any cgroup hierarchy - subsys list, flags,
* release_agent_path and so on. Modifying requires both cgroup_mutex and
* cgroup_root_mutex. Readers can acquire either of the two. This is to
* break the following locking order cycle.
*
* A. cgroup_mutex -> cred_guard_mutex -> s_type->i_mutex_key -> namespace_sem
* B. namespace_sem -> cgroup_mutex
*
* B happens only through cgroup_show_options() and using cgroup_root_mutex
* breaks it.
*/
static DEFINE_MUTEX(cgroup_mutex);
static DEFINE_MUTEX(cgroup_root_mutex);
/*
* Generate an array of cgroup subsystem pointers. At boot time, this is
@ -953,6 +970,7 @@ static int rebind_subsystems(struct cgroupfs_root *root,
int i;
BUG_ON(!mutex_is_locked(&cgroup_mutex));
BUG_ON(!mutex_is_locked(&cgroup_root_mutex));
removed_bits = root->actual_subsys_bits & ~final_bits;
added_bits = final_bits & ~root->actual_subsys_bits;
@ -1043,7 +1061,7 @@ static int cgroup_show_options(struct seq_file *seq, struct vfsmount *vfs)
struct cgroupfs_root *root = vfs->mnt_sb->s_fs_info;
struct cgroup_subsys *ss;
mutex_lock(&cgroup_mutex);
mutex_lock(&cgroup_root_mutex);
for_each_subsys(root, ss)
seq_printf(seq, ",%s", ss->name);
if (test_bit(ROOT_NOPREFIX, &root->flags))
@ -1054,7 +1072,7 @@ static int cgroup_show_options(struct seq_file *seq, struct vfsmount *vfs)
seq_puts(seq, ",clone_children");
if (strlen(root->name))
seq_printf(seq, ",name=%s", root->name);
mutex_unlock(&cgroup_mutex);
mutex_unlock(&cgroup_root_mutex);
return 0;
}
@ -1269,6 +1287,7 @@ static int cgroup_remount(struct super_block *sb, int *flags, char *data)
mutex_lock(&cgrp->dentry->d_inode->i_mutex);
mutex_lock(&cgroup_mutex);
mutex_lock(&cgroup_root_mutex);
/* See what subsystems are wanted */
ret = parse_cgroupfs_options(data, &opts);
@ -1297,6 +1316,7 @@ static int cgroup_remount(struct super_block *sb, int *flags, char *data)
out_unlock:
kfree(opts.release_agent);
kfree(opts.name);
mutex_unlock(&cgroup_root_mutex);
mutex_unlock(&cgroup_mutex);
mutex_unlock(&cgrp->dentry->d_inode->i_mutex);
return ret;
@ -1481,6 +1501,7 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
int ret = 0;
struct super_block *sb;
struct cgroupfs_root *new_root;
struct inode *inode;
/* First find the desired set of subsystems */
mutex_lock(&cgroup_mutex);
@ -1514,7 +1535,6 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
/* We used the new root structure, so this is a new hierarchy */
struct list_head tmp_cg_links;
struct cgroup *root_cgrp = &root->top_cgroup;
struct inode *inode;
struct cgroupfs_root *existing_root;
const struct cred *cred;
int i;
@ -1528,18 +1548,14 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
mutex_lock(&inode->i_mutex);
mutex_lock(&cgroup_mutex);
mutex_lock(&cgroup_root_mutex);
if (strlen(root->name)) {
/* Check for name clashes with existing mounts */
for_each_active_root(existing_root) {
if (!strcmp(existing_root->name, root->name)) {
ret = -EBUSY;
mutex_unlock(&cgroup_mutex);
mutex_unlock(&inode->i_mutex);
goto drop_new_super;
}
}
}
/* Check for name clashes with existing mounts */
ret = -EBUSY;
if (strlen(root->name))
for_each_active_root(existing_root)
if (!strcmp(existing_root->name, root->name))
goto unlock_drop;
/*
* We're accessing css_set_count without locking
@ -1549,18 +1565,13 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
* have some link structures left over
*/
ret = allocate_cg_links(css_set_count, &tmp_cg_links);
if (ret) {
mutex_unlock(&cgroup_mutex);
mutex_unlock(&inode->i_mutex);
goto drop_new_super;
}
if (ret)
goto unlock_drop;
ret = rebind_subsystems(root, root->subsys_bits);
if (ret == -EBUSY) {
mutex_unlock(&cgroup_mutex);
mutex_unlock(&inode->i_mutex);
free_cg_links(&tmp_cg_links);
goto drop_new_super;
goto unlock_drop;
}
/*
* There must be no failure case after here, since rebinding
@ -1599,6 +1610,7 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
cred = override_creds(&init_cred);
cgroup_populate_dir(root_cgrp);
revert_creds(cred);
mutex_unlock(&cgroup_root_mutex);
mutex_unlock(&cgroup_mutex);
mutex_unlock(&inode->i_mutex);
} else {
@ -1615,6 +1627,10 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
kfree(opts.name);
return dget(sb->s_root);
unlock_drop:
mutex_unlock(&cgroup_root_mutex);
mutex_unlock(&cgroup_mutex);
mutex_unlock(&inode->i_mutex);
drop_new_super:
deactivate_locked_super(sb);
drop_modules:
@ -1639,6 +1655,7 @@ static void cgroup_kill_sb(struct super_block *sb) {
BUG_ON(!list_empty(&cgrp->sibling));
mutex_lock(&cgroup_mutex);
mutex_lock(&cgroup_root_mutex);
/* Rebind all subsystems back to the default hierarchy */
ret = rebind_subsystems(root, 0);
@ -1664,6 +1681,7 @@ static void cgroup_kill_sb(struct super_block *sb) {
root_count--;
}
mutex_unlock(&cgroup_root_mutex);
mutex_unlock(&cgroup_mutex);
kill_litter_super(sb);
@ -2311,7 +2329,9 @@ static int cgroup_release_agent_write(struct cgroup *cgrp, struct cftype *cft,
return -EINVAL;
if (!cgroup_lock_live_group(cgrp))
return -ENODEV;
mutex_lock(&cgroup_root_mutex);
strcpy(cgrp->root->release_agent_path, buffer);
mutex_unlock(&cgroup_root_mutex);
cgroup_unlock();
return 0;
}