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Subject[PATCH 01/10] cgroup: add cgroup_root_mutex
Date
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>
Cc: Oleg Nesterov <oleg@redhat.com>
---
kernel/cgroup.c | 64 ++++++++++++++++++++++++++++++++++++-------------------
1 files changed, 42 insertions(+), 22 deletions(-)
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 453100a..efa5886 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -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);
@@ -2308,7 +2326,9 @@ 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;
}
--
1.7.3.1


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