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Subject[PATCH 4/4] cgroup: convert from per-cgroup id to per-subsys id
Date
Currently, cgrp->id is only used to look up css's.  As cgroup and
css's lifetimes is now decoupled, it should be made per-subsystem
and moved to css->css_id so that lookups are successful until the
target css is released.

Signed-off-by: Jianyu Zhan <nasa4836@gmail.com>
---
include/linux/cgroup.h | 26 +++++++--------
kernel/cgroup.c | 87 ++++++++++++++++++++++++--------------------------
2 files changed, 55 insertions(+), 58 deletions(-)

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index de31b2a..66085bd 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -62,6 +62,16 @@ struct cgroup_subsys_state {
/* the parent css */
struct cgroup_subsys_state *parent;

+ /*
+ * per css id
+ *
+ * The css id of cgrp_dfl_root for each subsys is always 0, and
+ * a new css will be assigned with a smallest available ID.
+ *
+ * Allocating/Removing ID must be protected by cgroup_mutex.
+ */
+ int css_id;
+
unsigned long flags;

/* percpu_ref killing and RCU release */
@@ -141,16 +151,6 @@ enum {
struct cgroup {
unsigned long flags; /* "unsigned long" so bitops work */

- /*
- * idr allocated in-hierarchy ID.
- *
- * The ID of the root cgroup is always 0, and a new cgroup
- * will be assigned with a smallest available ID.
- *
- * Allocating/Removing ID must be protected by cgroup_mutex.
- */
- int id;
-
/* the number of attached css's */
int nr_css;

@@ -329,9 +329,6 @@ struct cgroup_root {
/* Hierarchy-specific flags */
unsigned long flags;

- /* IDs for cgroups in this hierarchy */
- struct idr cgroup_idr;
-
/* The path to use for release notifications. */
char release_agent_path[PATH_MAX];

@@ -655,6 +652,9 @@ struct cgroup_subsys {
/* link to parent, protected by cgroup_lock() */
struct cgroup_root *root;

+ /* IDs for css'es of this subsys */
+ struct idr css_idr;
+
/*
* List of cftypes. Each entry is the first entry of an array
* terminated by zero length name.
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 80c1cf3..f41cf8b 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -838,7 +838,6 @@ static void cgroup_free_root(struct cgroup_root *root)
/* hierarhcy ID shoulid already have been released */
WARN_ON_ONCE(root->hierarchy_id);

- idr_destroy(&root->cgroup_idr);
kfree(root);
}
}
@@ -1050,17 +1049,6 @@ static void cgroup_put(struct cgroup *cgrp)
if (WARN_ON_ONCE(cgrp->parent && !cgroup_is_dead(cgrp)))
return;

- /*
- * XXX: cgrp->id is only used to look up css's. As cgroup and
- * css's lifetimes will be decoupled, it should be made
- * per-subsystem and moved to css->id so that lookups are
- * successful until the target css is released.
- */
- mutex_lock(&cgroup_mutex);
- idr_remove(&cgrp->root->cgroup_idr, cgrp->id);
- mutex_unlock(&cgroup_mutex);
- cgrp->id = -1;
-
call_rcu(&cgrp->rcu_head, cgroup_free_rcu);
}

@@ -1512,7 +1500,6 @@ static void init_cgroup_root(struct cgroup_root *root,
atomic_set(&root->nr_cgrps, 1);
cgrp->root = root;
init_cgroup_housekeeping(cgrp);
- idr_init(&root->cgroup_idr);

root->flags = opts->flags;
if (opts->release_agent)
@@ -1533,11 +1520,6 @@ static int cgroup_setup_root(struct cgroup_root *root, unsigned long ss_mask)
lockdep_assert_held(&cgroup_tree_mutex);
lockdep_assert_held(&cgroup_mutex);

- ret = idr_alloc(&root->cgroup_idr, root_cgrp, 0, 1, GFP_KERNEL);
- if (ret < 0)
- goto out;
- root_cgrp->id = ret;
-
/*
* We're accessing css_set_count without locking css_set_rwsem here,
* but that's OK - it can only be increased by someone holding
@@ -4056,6 +4038,11 @@ static void css_free_work_fn(struct work_struct *work)

css->ss->css_free(css);
cgroup_put(cgrp);
+
+ mutex_lock(&cgroup_mutex);
+ idr_remove(&css->ss->css_idr, css->css_id);
+ mutex_unlock(&cgroup_mutex);
+ css->css_id = -1;
}

static void css_free_rcu_fn(struct rcu_head *rcu_head)
@@ -4152,9 +4139,17 @@ static int create_css(struct cgroup *cgrp, struct cgroup_subsys *ss)
if (IS_ERR(css))
return PTR_ERR(css);

+ /*
+ * Temporarily set the pointer to NULL, so idr_find() won't return
+ * a half-baked css.
+ */
+ css->css_id = idr_alloc(&ss->css_idr, NULL, 1, 0, GFP_KERNEL);
+ if (css->css_id < 0)
+ goto err_free_css;
+
err = percpu_ref_init(&css->refcnt, css_release);
if (err)
- goto err_free_css;
+ goto err_free_id;

init_css(css, ss, cgrp);

@@ -4166,6 +4161,12 @@ static int create_css(struct cgroup *cgrp, struct cgroup_subsys *ss)
if (err)
goto err_clear_dir;

+ /*
+ * @css is now fully operational.
+ * Nothing can fail from this point on.
+ */
+ idr_replace(&ss->css_idr, css, css->css_id);
+
cgroup_get(cgrp);
css_get(css->parent);

@@ -4184,6 +4185,8 @@ err_clear_dir:
cgroup_clear_dir(css->cgroup, 1 << css->ss->id);
err_free_percpu_ref:
percpu_ref_cancel_init(&css->refcnt);
+err_free_id:
+ idr_remove(&ss->css_idr, css->css_id);
err_free_css:
ss->css_free(css);
return err;
@@ -4223,16 +4226,6 @@ static long cgroup_create(struct cgroup *parent, const char *name,
goto err_unlock_tree;
}

- /*
- * Temporarily set the pointer to NULL, so idr_find() won't return
- * a half-baked cgroup.
- */
- cgrp->id = idr_alloc(&root->cgroup_idr, NULL, 1, 0, GFP_KERNEL);
- if (cgrp->id < 0) {
- err = -ENOMEM;
- goto err_unlock;
- }
-
init_cgroup_housekeeping(cgrp);

cgrp->parent = parent;
@@ -4249,7 +4242,7 @@ static long cgroup_create(struct cgroup *parent, const char *name,
kn = kernfs_create_dir(parent->kn, name, mode, cgrp);
if (IS_ERR(kn)) {
err = PTR_ERR(kn);
- goto err_free_id;
+ goto err_unlock;
}
cgrp->kn = kn;

@@ -4266,12 +4259,6 @@ static long cgroup_create(struct cgroup *parent, const char *name,
atomic_inc(&root->nr_cgrps);
cgroup_get(parent);

- /*
- * @cgrp is now fully operational. If something fails after this
- * point, it'll be released via the normal destruction path.
- */
- idr_replace(&root->cgroup_idr, cgrp, cgrp->id);
-
err = cgroup_kn_set_ugid(kn);
if (err)
goto err_destroy;
@@ -4303,8 +4290,6 @@ static long cgroup_create(struct cgroup *parent, const char *name,

return 0;

-err_free_id:
- idr_remove(&root->cgroup_idr, cgrp->id);
err_unlock:
mutex_unlock(&cgroup_mutex);
err_unlock_tree:
@@ -4617,6 +4602,7 @@ static void __init cgroup_init_subsys(struct cgroup_subsys *ss)
mutex_lock(&cgroup_tree_mutex);
mutex_lock(&cgroup_mutex);

+ idr_init(&ss->css_idr);
INIT_LIST_HEAD(&ss->cfts);

/* Create the root cgroup state for this subsystem */
@@ -4707,9 +4693,25 @@ int __init cgroup_init(void)
mutex_unlock(&cgroup_tree_mutex);

for_each_subsys(ss, ssid) {
+ struct cgroup_subsys_state *css;
+ int id;
+
if (!ss->early_init)
cgroup_init_subsys(ss);

+ /*
+ * mm_init() runs lately after cgroup_init_early(), thus
+ * the world isn't set up yet for idr_alloc() to run, so
+ * we have to defer the id allocation to this point.
+ *
+ * And we don't gracefully handle early failure.
+ */
+ css = init_css_set.subsys[ss->id];
+ id = idr_alloc(&ss->css_idr, css, 0, 1, GFP_KERNEL);
+ if (id < 0)
+ BUG();
+ css->css_id = id;
+
list_add_tail(&init_css_set.e_cset_node[ssid],
&cgrp_dfl_root.cgrp.e_csets[ssid]);

@@ -5160,7 +5162,7 @@ struct cgroup_subsys_state *css_tryget_from_dir(struct dentry *dentry,
*/
int css_to_id(struct cgroup_subsys_state *css)
{
- return css->cgroup->id;
+ return css->css_id;
}

/**
@@ -5173,14 +5175,9 @@ int css_to_id(struct cgroup_subsys_state *css)
*/
struct cgroup_subsys_state *css_from_id(int id, struct cgroup_subsys *ss)
{
- struct cgroup *cgrp;
-
cgroup_assert_mutexes_or_rcu_locked();

- cgrp = idr_find(&ss->root->cgroup_idr, id);
- if (cgrp)
- return cgroup_css(cgrp, ss);
- return NULL;
+ return idr_find(&ss->css_idr, id);
}

#ifdef CONFIG_CGROUP_DEBUG
--
2.0.0-rc0


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