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cgroup: remove cgroup_root_mutex
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cgroup_root_mutex was added to avoid deadlock involving namespace_sem
via cgroup_show_options().  It added a lot of overhead for the small
purpose of it and, because it's nested under cgroup_mutex, it has very
limited usefulness.  The previous patch made cgroup_show_options() not
use cgroup_root_mutex, so nobody needs it anymore.  Remove it.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
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Tejun Heo committed Feb 8, 2014
1 parent 69e943b commit 3417ae1
Showing 1 changed file with 1 addition and 41 deletions.
42 changes: 1 addition & 41 deletions kernel/cgroup.c
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Original file line number Diff line number Diff line change
Expand Up @@ -70,18 +70,6 @@
/*
* 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.
*/
#ifdef CONFIG_PROVE_RCU
DEFINE_MUTEX(cgroup_mutex);
Expand All @@ -90,8 +78,6 @@ EXPORT_SYMBOL_GPL(cgroup_mutex); /* only for lockdep */
static DEFINE_MUTEX(cgroup_mutex);
#endif

static DEFINE_MUTEX(cgroup_root_mutex);

/*
* Protects cgroup_subsys->release_agent_path. Modifying it also requires
* cgroup_mutex. Reading requires either cgroup_mutex or this spinlock.
Expand All @@ -103,14 +89,6 @@ static DEFINE_SPINLOCK(release_agent_path_lock);
lockdep_is_held(&cgroup_mutex), \
"cgroup_mutex or RCU read lock required");

#ifdef CONFIG_LOCKDEP
#define cgroup_assert_mutex_or_root_locked() \
WARN_ON_ONCE(debug_locks && (!lockdep_is_held(&cgroup_mutex) && \
!lockdep_is_held(&cgroup_root_mutex)))
#else
#define cgroup_assert_mutex_or_root_locked() do { } while (0)
#endif

/*
* cgroup destruction makes heavy use of work items and there can be a lot
* of concurrent destructions. Use a separate workqueue so that cgroup
Expand Down Expand Up @@ -154,11 +132,7 @@ static struct cgroup * const cgroup_dummy_top = &cgroup_dummy_root.top_cgroup;
static LIST_HEAD(cgroup_roots);
static int cgroup_root_count;

/*
* Hierarchy ID allocation and mapping. It follows the same exclusion
* rules as other root ops - both cgroup_mutex and cgroup_root_mutex for
* writes, either for reads.
*/
/* hierarchy ID allocation and mapping, protected by cgroup_mutex */
static DEFINE_IDR(cgroup_hierarchy_idr);

static struct cgroup_name root_cgroup_name = { .name = "/" };
Expand Down Expand Up @@ -973,7 +947,6 @@ static int rebind_subsystems(struct cgroupfs_root *root,
int i, ret;

BUG_ON(!mutex_is_locked(&cgroup_mutex));
BUG_ON(!mutex_is_locked(&cgroup_root_mutex));

/* Check that any added subsystems are currently free */
for_each_subsys(ss, i)
Expand Down Expand Up @@ -1246,7 +1219,6 @@ 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);
Expand Down Expand Up @@ -1288,7 +1260,6 @@ 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;
Expand Down Expand Up @@ -1331,7 +1302,6 @@ static int cgroup_init_root_id(struct cgroupfs_root *root, int start, int end)
int id;

lockdep_assert_held(&cgroup_mutex);
lockdep_assert_held(&cgroup_root_mutex);

id = idr_alloc_cyclic(&cgroup_hierarchy_idr, root, start, end,
GFP_KERNEL);
Expand All @@ -1345,7 +1315,6 @@ static int cgroup_init_root_id(struct cgroupfs_root *root, int start, int end)
static void cgroup_exit_root_id(struct cgroupfs_root *root)
{
lockdep_assert_held(&cgroup_mutex);
lockdep_assert_held(&cgroup_root_mutex);

if (root->hierarchy_id) {
idr_remove(&cgroup_hierarchy_idr, root->hierarchy_id);
Expand Down Expand Up @@ -1524,7 +1493,6 @@ 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);

root_cgrp->id = idr_alloc(&root->cgroup_idr, root_cgrp,
0, 1, GFP_KERNEL);
Expand Down Expand Up @@ -1597,7 +1565,6 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
BUG_ON(!list_empty(&root_cgrp->children));
BUG_ON(root->number_of_cgroups != 1);

mutex_unlock(&cgroup_root_mutex);
mutex_unlock(&cgroup_mutex);
mutex_unlock(&inode->i_mutex);
} else {
Expand Down Expand Up @@ -1628,7 +1595,6 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
revert_creds(cred);
unlock_drop:
cgroup_exit_root_id(root);
mutex_unlock(&cgroup_root_mutex);
mutex_unlock(&cgroup_mutex);
mutex_unlock(&inode->i_mutex);
drop_new_super:
Expand All @@ -1653,7 +1619,6 @@ static void cgroup_kill_sb(struct super_block *sb)

mutex_lock(&cgrp->dentry->d_inode->i_mutex);
mutex_lock(&cgroup_mutex);
mutex_lock(&cgroup_root_mutex);

/* Rebind all subsystems back to the default hierarchy */
if (root->flags & CGRP_ROOT_SUBSYS_BOUND) {
Expand Down Expand Up @@ -1682,7 +1647,6 @@ static void cgroup_kill_sb(struct super_block *sb)

cgroup_exit_root_id(root);

mutex_unlock(&cgroup_root_mutex);
mutex_unlock(&cgroup_mutex);
mutex_unlock(&cgrp->dentry->d_inode->i_mutex);

Expand Down Expand Up @@ -2193,11 +2157,9 @@ static int cgroup_release_agent_write(struct cgroup_subsys_state *css,
return -EINVAL;
if (!cgroup_lock_live_group(css->cgroup))
return -ENODEV;
mutex_lock(&cgroup_root_mutex);
spin_lock(&release_agent_path_lock);
strcpy(css->cgroup->root->release_agent_path, buffer);
spin_unlock(&release_agent_path_lock);
mutex_unlock(&cgroup_root_mutex);
mutex_unlock(&cgroup_mutex);
return 0;
}
Expand Down Expand Up @@ -4588,7 +4550,6 @@ int __init cgroup_init(void)

/* allocate id for the dummy hierarchy */
mutex_lock(&cgroup_mutex);
mutex_lock(&cgroup_root_mutex);

/* Add init_css_set to the hash table */
key = css_set_hash(init_css_set.subsys);
Expand All @@ -4600,7 +4561,6 @@ int __init cgroup_init(void)
0, 1, GFP_KERNEL);
BUG_ON(err < 0);

mutex_unlock(&cgroup_root_mutex);
mutex_unlock(&cgroup_mutex);

cgroup_kobj = kobject_create_and_add("cgroup", fs_kobj);
Expand Down

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