cgroup: drop task_lock() protection around task->cgroups

[mirror_ubuntu-zesty-kernel.git] / include / linux / cgroup.h

#ifndef _LINUX_CGROUP_H
#define _LINUX_CGROUP_H
/*
 *  cgroup interface
 *
 *  Copyright (C) 2003 BULL SA
 *  Copyright (C) 2004-2006 Silicon Graphics, Inc.
 *
 */

#include <linux/sched.h>
#include <linux/cpumask.h>
#include <linux/nodemask.h>
#include <linux/rcupdate.h>
#include <linux/rculist.h>
#include <linux/cgroupstats.h>
#include <linux/rwsem.h>
#include <linux/idr.h>
#include <linux/workqueue.h>
#include <linux/fs.h>
#include <linux/percpu-refcount.h>
#include <linux/seq_file.h>
#include <linux/kernfs.h>

#ifdef CONFIG_CGROUPS

struct cgroupfs_root;
struct cgroup_subsys;
struct inode;
struct cgroup;

extern int cgroup_init_early(void);
extern int cgroup_init(void);
extern void cgroup_fork(struct task_struct *p);
extern void cgroup_post_fork(struct task_struct *p);
extern void cgroup_exit(struct task_struct *p, int run_callbacks);
extern int cgroupstats_build(struct cgroupstats *stats,
                                struct dentry *dentry);

extern int proc_cgroup_show(struct seq_file *, void *);

/* define the enumeration of all cgroup subsystems */
#define SUBSYS(_x) _x ## _cgrp_id,
enum cgroup_subsys_id {
#include <linux/cgroup_subsys.h>
        CGROUP_SUBSYS_COUNT,
};
#undef SUBSYS

/* Per-subsystem/per-cgroup state maintained by the system. */
struct cgroup_subsys_state {
        /* the cgroup that this css is attached to */
        struct cgroup *cgroup;

        /* the cgroup subsystem that this css is attached to */
        struct cgroup_subsys *ss;

        /* reference count - access via css_[try]get() and css_put() */
        struct percpu_ref refcnt;

        /* the parent css */
        struct cgroup_subsys_state *parent;

        unsigned long flags;

        /* percpu_ref killing and RCU release */
        struct rcu_head rcu_head;
        struct work_struct destroy_work;
};

/* bits in struct cgroup_subsys_state flags field */
enum {
        CSS_ROOT        = (1 << 0), /* this CSS is the root of the subsystem */
        CSS_ONLINE      = (1 << 1), /* between ->css_online() and ->css_offline() */
};

/**
 * css_get - obtain a reference on the specified css
 * @css: target css
 *
 * The caller must already have a reference.
 */
static inline void css_get(struct cgroup_subsys_state *css)
{
        /* We don't need to reference count the root state */
        if (!(css->flags & CSS_ROOT))
                percpu_ref_get(&css->refcnt);
}

/**
 * css_tryget - try to obtain a reference on the specified css
 * @css: target css
 *
 * Obtain a reference on @css if it's alive.  The caller naturally needs to
 * ensure that @css is accessible but doesn't have to be holding a
 * reference on it - IOW, RCU protected access is good enough for this
 * function.  Returns %true if a reference count was successfully obtained;
 * %false otherwise.
 */
static inline bool css_tryget(struct cgroup_subsys_state *css)
{
        if (css->flags & CSS_ROOT)
                return true;
        return percpu_ref_tryget(&css->refcnt);
}

/**
 * css_put - put a css reference
 * @css: target css
 *
 * Put a reference obtained via css_get() and css_tryget().
 */
static inline void css_put(struct cgroup_subsys_state *css)
{
        if (!(css->flags & CSS_ROOT))
                percpu_ref_put(&css->refcnt);
}

/* bits in struct cgroup flags field */
enum {
        /* Control Group is dead */
        CGRP_DEAD,
        /*
         * Control Group has previously had a child cgroup or a task,
         * but no longer (only if CGRP_NOTIFY_ON_RELEASE is set)
         */
        CGRP_RELEASABLE,
        /* Control Group requires release notifications to userspace */
        CGRP_NOTIFY_ON_RELEASE,
        /*
         * Clone the parent's configuration when creating a new child
         * cpuset cgroup.  For historical reasons, this option can be
         * specified at mount time and thus is implemented here.
         */
        CGRP_CPUSET_CLONE_CHILDREN,
        /* see the comment above CGRP_ROOT_SANE_BEHAVIOR for details */
        CGRP_SANE_BEHAVIOR,
};

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;

        atomic_t refcnt;

        /*
         * We link our 'sibling' struct into our parent's 'children'.
         * Our children link their 'sibling' into our 'children'.
         */
        struct list_head sibling;       /* my parent's children */
        struct list_head children;      /* my children */

        struct cgroup *parent;          /* my parent */
        struct kernfs_node *kn;         /* cgroup kernfs entry */

        /*
         * Monotonically increasing unique serial number which defines a
         * uniform order among all cgroups.  It's guaranteed that all
         * ->children lists are in the ascending order of ->serial_nr.
         * It's used to allow interrupting and resuming iterations.
         */
        u64 serial_nr;

        /* Private pointers for each registered subsystem */
        struct cgroup_subsys_state __rcu *subsys[CGROUP_SUBSYS_COUNT];

        struct cgroupfs_root *root;

        /*
         * List of cgrp_cset_links pointing at css_sets with tasks in this
         * cgroup.  Protected by css_set_lock.
         */
        struct list_head cset_links;

        /*
         * Linked list running through all cgroups that can
         * potentially be reaped by the release agent. Protected by
         * release_list_lock
         */
        struct list_head release_list;

        /*
         * list of pidlists, up to two for each namespace (one for procs, one
         * for tasks); created on demand.
         */
        struct list_head pidlists;
        struct mutex pidlist_mutex;

        /* dummy css with NULL ->ss, points back to this cgroup */
        struct cgroup_subsys_state dummy_css;

        /* For css percpu_ref killing and RCU-protected deletion */
        struct rcu_head rcu_head;
        struct work_struct destroy_work;
};

#define MAX_CGROUP_ROOT_NAMELEN 64

/* cgroupfs_root->flags */
enum {
        /*
         * Unfortunately, cgroup core and various controllers are riddled
         * with idiosyncrasies and pointless options.  The following flag,
         * when set, will force sane behavior - some options are forced on,
         * others are disallowed, and some controllers will change their
         * hierarchical or other behaviors.
         *
         * The set of behaviors affected by this flag are still being
         * determined and developed and the mount option for this flag is
         * prefixed with __DEVEL__.  The prefix will be dropped once we
         * reach the point where all behaviors are compatible with the
         * planned unified hierarchy, which will automatically turn on this
         * flag.
         *
         * The followings are the behaviors currently affected this flag.
         *
         * - Mount options "noprefix", "xattr", "clone_children",
         *   "release_agent" and "name" are disallowed.
         *
         * - When mounting an existing superblock, mount options should
         *   match.
         *
         * - Remount is disallowed.
         *
         * - rename(2) is disallowed.
         *
         * - "tasks" is removed.  Everything should be at process
         *   granularity.  Use "cgroup.procs" instead.
         *
         * - "cgroup.procs" is not sorted.  pids will be unique unless they
         *   got recycled inbetween reads.
         *
         * - "release_agent" and "notify_on_release" are removed.
         *   Replacement notification mechanism will be implemented.
         *
         * - "cgroup.clone_children" is removed.
         *
         * - cpuset: tasks will be kept in empty cpusets when hotplug happens
         *   and take masks of ancestors with non-empty cpus/mems, instead of
         *   being moved to an ancestor.
         *
         * - cpuset: a task can be moved into an empty cpuset, and again it
         *   takes masks of ancestors.
         *
         * - memcg: use_hierarchy is on by default and the cgroup file for
         *   the flag is not created.
         *
         * - blkcg: blk-throttle becomes properly hierarchical.
         */
        CGRP_ROOT_SANE_BEHAVIOR = (1 << 0),

        CGRP_ROOT_NOPREFIX      = (1 << 1), /* mounted subsystems have no named prefix */
        CGRP_ROOT_XATTR         = (1 << 2), /* supports extended attributes */

        /* mount options live below bit 16 */
        CGRP_ROOT_OPTION_MASK   = (1 << 16) - 1,
};

/*
 * A cgroupfs_root represents the root of a cgroup hierarchy, and may be
 * associated with a kernfs_root to form an active hierarchy.  This is
 * internal to cgroup core.  Don't access directly from controllers.
 */
struct cgroupfs_root {
        struct kernfs_root *kf_root;

        /* The bitmask of subsystems attached to this hierarchy */
        unsigned long subsys_mask;

        /* Unique id for this hierarchy. */
        int hierarchy_id;

        /* The root cgroup.  Root is destroyed on its release. */
        struct cgroup top_cgroup;

        /* Number of cgroups in the hierarchy, used only for /proc/cgroups */
        atomic_t nr_cgrps;

        /* A list running through the active hierarchies */
        struct list_head root_list;

        /* 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];

        /* The name for this hierarchy - may be empty */
        char name[MAX_CGROUP_ROOT_NAMELEN];
};

/*
 * A css_set is a structure holding pointers to a set of
 * cgroup_subsys_state objects. This saves space in the task struct
 * object and speeds up fork()/exit(), since a single inc/dec and a
 * list_add()/del() can bump the reference count on the entire cgroup
 * set for a task.
 */

struct css_set {

        /* Reference count */
        atomic_t refcount;

        /*
         * List running through all cgroup groups in the same hash
         * slot. Protected by css_set_lock
         */
        struct hlist_node hlist;

        /*
         * Lists running through all tasks using this cgroup group.
         * mg_tasks lists tasks which belong to this cset but are in the
         * process of being migrated out or in.  Protected by
         * css_set_rwsem, but, during migration, once tasks are moved to
         * mg_tasks, it can be read safely while holding cgroup_mutex.
         */
        struct list_head tasks;
        struct list_head mg_tasks;

        /*
         * List of cgrp_cset_links pointing at cgroups referenced from this
         * css_set.  Protected by css_set_lock.
         */
        struct list_head cgrp_links;

        /*
         * 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).
         */
        struct cgroup_subsys_state *subsys[CGROUP_SUBSYS_COUNT];

        /*
         * List of csets participating in the on-going migration either as
         * source or destination.  Protected by cgroup_mutex.
         */
        struct list_head mg_preload_node;
        struct list_head mg_node;

        /*
         * If this cset is acting as the source of migration the following
         * two fields are set.  mg_src_cgrp is the source cgroup of the
         * on-going migration and mg_dst_cset is the destination cset the
         * target tasks on this cset should be migrated to.  Protected by
         * cgroup_mutex.
         */
        struct cgroup *mg_src_cgrp;
        struct css_set *mg_dst_cset;

        /* For RCU-protected deletion */
        struct rcu_head rcu_head;
};

/*
 * struct cftype: handler definitions for cgroup control files
 *
 * When reading/writing to a file:
 *      - the cgroup to use is file->f_dentry->d_parent->d_fsdata
 *      - the 'cftype' of the file is file->f_dentry->d_fsdata
 */

/* cftype->flags */
enum {
        CFTYPE_ONLY_ON_ROOT     = (1 << 0),     /* only create on root cgrp */
        CFTYPE_NOT_ON_ROOT      = (1 << 1),     /* don't create on root cgrp */
        CFTYPE_INSANE           = (1 << 2),     /* don't create if sane_behavior */
        CFTYPE_NO_PREFIX        = (1 << 3),     /* (DON'T USE FOR NEW FILES) no subsys prefix */
};

#define MAX_CFTYPE_NAME         64

struct cftype {
        /*
         * By convention, the name should begin with the name of the
         * subsystem, followed by a period.  Zero length string indicates
         * end of cftype array.
         */
        char name[MAX_CFTYPE_NAME];
        int private;
        /*
         * If not 0, file mode is set to this value, otherwise it will
         * be figured out automatically
         */
        umode_t mode;

        /*
         * The maximum length of string, excluding trailing nul, that can
         * be passed to write_string.  If < PAGE_SIZE-1, PAGE_SIZE-1 is
         * assumed.
         */
        size_t max_write_len;

        /* CFTYPE_* flags */
        unsigned int flags;

        /*
         * Fields used for internal bookkeeping.  Initialized automatically
         * during registration.
         */
        struct cgroup_subsys *ss;       /* NULL for cgroup core files */
        struct list_head node;          /* anchored at ss->cfts */
        struct kernfs_ops *kf_ops;

        /*
         * read_u64() is a shortcut for the common case of returning a
         * single integer. Use it in place of read()
         */
        u64 (*read_u64)(struct cgroup_subsys_state *css, struct cftype *cft);
        /*
         * read_s64() is a signed version of read_u64()
         */
        s64 (*read_s64)(struct cgroup_subsys_state *css, struct cftype *cft);

        /* generic seq_file read interface */
        int (*seq_show)(struct seq_file *sf, void *v);

        /* optional ops, implement all or none */
        void *(*seq_start)(struct seq_file *sf, loff_t *ppos);
        void *(*seq_next)(struct seq_file *sf, void *v, loff_t *ppos);
        void (*seq_stop)(struct seq_file *sf, void *v);

        /*
         * write_u64() is a shortcut for the common case of accepting
         * a single integer (as parsed by simple_strtoull) from
         * userspace. Use in place of write(); return 0 or error.
         */
        int (*write_u64)(struct cgroup_subsys_state *css, struct cftype *cft,
                         u64 val);
        /*
         * write_s64() is a signed version of write_u64()
         */
        int (*write_s64)(struct cgroup_subsys_state *css, struct cftype *cft,
                         s64 val);

        /*
         * write_string() is passed a nul-terminated kernelspace
         * buffer of maximum length determined by max_write_len.
         * Returns 0 or -ve error code.
         */
        int (*write_string)(struct cgroup_subsys_state *css, struct cftype *cft,
                            const char *buffer);
        /*
         * trigger() callback can be used to get some kick from the
         * userspace, when the actual string written is not important
         * at all. The private field can be used to determine the
         * kick type for multiplexing.
         */
        int (*trigger)(struct cgroup_subsys_state *css, unsigned int event);

#ifdef CONFIG_DEBUG_LOCK_ALLOC
        struct lock_class_key   lockdep_key;
#endif
};

/*
 * See the comment above CGRP_ROOT_SANE_BEHAVIOR for details.  This
 * function can be called as long as @cgrp is accessible.
 */
static inline bool cgroup_sane_behavior(const struct cgroup *cgrp)
{
        return cgrp->root->flags & CGRP_ROOT_SANE_BEHAVIOR;
}

/* no synchronization, the result can only be used as a hint */
static inline bool cgroup_has_tasks(struct cgroup *cgrp)
{
        return !list_empty(&cgrp->cset_links);
}

/* returns ino associated with a cgroup, 0 indicates unmounted root */
static inline ino_t cgroup_ino(struct cgroup *cgrp)
{
        if (cgrp->kn)
                return cgrp->kn->ino;
        else
                return 0;
}

static inline struct cftype *seq_cft(struct seq_file *seq)
{
        struct kernfs_open_file *of = seq->private;

        return of->kn->priv;
}

struct cgroup_subsys_state *seq_css(struct seq_file *seq);

/*
 * Name / path handling functions.  All are thin wrappers around the kernfs
 * counterparts and can be called under any context.
 */

static inline int cgroup_name(struct cgroup *cgrp, char *buf, size_t buflen)
{
        /* dummy_top doesn't have a kn associated */
        if (cgrp->kn)
                return kernfs_name(cgrp->kn, buf, buflen);
        else
                return strlcpy(buf, "/", buflen);
}

static inline char * __must_check cgroup_path(struct cgroup *cgrp, char *buf,
                                              size_t buflen)
{
        /* dummy_top doesn't have a kn associated */
        if (cgrp->kn)
                return kernfs_path(cgrp->kn, buf, buflen);
        strlcpy(buf, "/", buflen);
        return (buflen <= 2) ? NULL : buf;
}

static inline void pr_cont_cgroup_name(struct cgroup *cgrp)
{
        /* dummy_top doesn't have a kn associated */
        if (cgrp->kn)
                pr_cont_kernfs_name(cgrp->kn);
        else
                pr_cont("/");
}

static inline void pr_cont_cgroup_path(struct cgroup *cgrp)
{
        /* dummy_top doesn't have a kn associated */
        if (cgrp->kn)
                pr_cont_kernfs_path(cgrp->kn);
        else
                pr_cont("/");
}

char *task_cgroup_path(struct task_struct *task, char *buf, size_t buflen);

int cgroup_add_cftypes(struct cgroup_subsys *ss, struct cftype *cfts);
int cgroup_rm_cftypes(struct cftype *cfts);

bool cgroup_is_descendant(struct cgroup *cgrp, struct cgroup *ancestor);

/*
 * Control Group taskset, used to pass around set of tasks to cgroup_subsys
 * methods.
 */
struct cgroup_taskset;
struct task_struct *cgroup_taskset_first(struct cgroup_taskset *tset);
struct task_struct *cgroup_taskset_next(struct cgroup_taskset *tset);

/**
 * cgroup_taskset_for_each - iterate cgroup_taskset
 * @task: the loop cursor
 * @tset: taskset to iterate
 */
#define cgroup_taskset_for_each(task, tset)                             \
        for ((task) = cgroup_taskset_first((tset)); (task);             \
             (task) = cgroup_taskset_next((tset)))

/*
 * Control Group subsystem type.
 * See Documentation/cgroups/cgroups.txt for details
 */

struct cgroup_subsys {
        struct cgroup_subsys_state *(*css_alloc)(struct cgroup_subsys_state *parent_css);
        int (*css_online)(struct cgroup_subsys_state *css);
        void (*css_offline)(struct cgroup_subsys_state *css);
        void (*css_free)(struct cgroup_subsys_state *css);

        int (*can_attach)(struct cgroup_subsys_state *css,
                          struct cgroup_taskset *tset);
        void (*cancel_attach)(struct cgroup_subsys_state *css,
                              struct cgroup_taskset *tset);
        void (*attach)(struct cgroup_subsys_state *css,
                       struct cgroup_taskset *tset);
        void (*fork)(struct task_struct *task);
        void (*exit)(struct cgroup_subsys_state *css,
                     struct cgroup_subsys_state *old_css,
                     struct task_struct *task);
        void (*bind)(struct cgroup_subsys_state *root_css);

        int disabled;
        int early_init;

        /*
         * If %false, this subsystem is properly hierarchical -
         * configuration, resource accounting and restriction on a parent
         * cgroup cover those of its children.  If %true, hierarchy support
         * is broken in some ways - some subsystems ignore hierarchy
         * completely while others are only implemented half-way.
         *
         * It's now disallowed to create nested cgroups if the subsystem is
         * broken and cgroup core will emit a warning message on such
         * cases.  Eventually, all subsystems will be made properly
         * hierarchical and this will go away.
         */
        bool broken_hierarchy;
        bool warned_broken_hierarchy;

        /* the following two fields are initialized automtically during boot */
        int id;
#define MAX_CGROUP_TYPE_NAMELEN 32
        const char *name;

        /* link to parent, protected by cgroup_lock() */
        struct cgroupfs_root *root;

        /*
         * List of cftypes.  Each entry is the first entry of an array
         * terminated by zero length name.
         */
        struct list_head cfts;

        /* base cftypes, automatically registered with subsys itself */
        struct cftype *base_cftypes;
};

#define SUBSYS(_x) extern struct cgroup_subsys _x ## _cgrp_subsys;
#include <linux/cgroup_subsys.h>
#undef SUBSYS

/**
 * css_parent - find the parent css
 * @css: the target cgroup_subsys_state
 *
 * Return the parent css of @css.  This function is guaranteed to return
 * non-NULL parent as long as @css isn't the root.
 */
static inline
struct cgroup_subsys_state *css_parent(struct cgroup_subsys_state *css)
{
        return css->parent;
}

/**
 * task_css_set_check - obtain a task's css_set with extra access conditions
 * @task: the task to obtain css_set for
 * @__c: extra condition expression to be passed to rcu_dereference_check()
 *
 * A task's css_set is RCU protected, initialized and exited while holding
 * task_lock(), and can only be modified while holding both cgroup_mutex
 * and task_lock() while the task is alive.  This macro verifies that the
 * caller is inside proper critical section and returns @task's css_set.
 *
 * The caller can also specify additional allowed conditions via @__c, such
 * as locks used during the cgroup_subsys::attach() methods.
 */
#ifdef CONFIG_PROVE_RCU
extern struct mutex cgroup_mutex;
extern struct rw_semaphore css_set_rwsem;
#define task_css_set_check(task, __c)                                   \
        rcu_dereference_check((task)->cgroups,                          \
                lockdep_is_held(&cgroup_mutex) ||                       \
                lockdep_is_held(&css_set_rwsem) ||                      \
                ((task)->flags & PF_EXITING) || (__c))
#else
#define task_css_set_check(task, __c)                                   \
        rcu_dereference((task)->cgroups)
#endif

/**
 * task_css_check - obtain css for (task, subsys) w/ extra access conds
 * @task: the target task
 * @subsys_id: the target subsystem ID
 * @__c: extra condition expression to be passed to rcu_dereference_check()
 *
 * Return the cgroup_subsys_state for the (@task, @subsys_id) pair.  The
 * synchronization rules are the same as task_css_set_check().
 */
#define task_css_check(task, subsys_id, __c)                            \
        task_css_set_check((task), (__c))->subsys[(subsys_id)]

/**
 * task_css_set - obtain a task's css_set
 * @task: the task to obtain css_set for
 *
 * See task_css_set_check().
 */
static inline struct css_set *task_css_set(struct task_struct *task)
{
        return task_css_set_check(task, false);
}

/**
 * task_css - obtain css for (task, subsys)
 * @task: the target task
 * @subsys_id: the target subsystem ID
 *
 * See task_css_check().
 */
static inline struct cgroup_subsys_state *task_css(struct task_struct *task,
                                                   int subsys_id)
{
        return task_css_check(task, subsys_id, false);
}

static inline struct cgroup *task_cgroup(struct task_struct *task,
                                         int subsys_id)
{
        return task_css(task, subsys_id)->cgroup;
}

struct cgroup_subsys_state *css_next_child(struct cgroup_subsys_state *pos,
                                           struct cgroup_subsys_state *parent);

struct cgroup_subsys_state *css_from_id(int id, struct cgroup_subsys *ss);

/**
 * css_for_each_child - iterate through children of a css
 * @pos: the css * to use as the loop cursor
 * @parent: css whose children to walk
 *
 * Walk @parent's children.  Must be called under rcu_read_lock().  A child
 * css which hasn't finished ->css_online() or already has finished
 * ->css_offline() may show up during traversal and it's each subsystem's
 * responsibility to verify that each @pos is alive.
 *
 * If a subsystem synchronizes against the parent in its ->css_online() and
 * before starting iterating, a css which finished ->css_online() is
 * guaranteed to be visible in the future iterations.
 *
 * It is allowed to temporarily drop RCU read lock during iteration.  The
 * caller is responsible for ensuring that @pos remains accessible until
 * the start of the next iteration by, for example, bumping the css refcnt.
 */
#define css_for_each_child(pos, parent)                                 \
        for ((pos) = css_next_child(NULL, (parent)); (pos);             \
             (pos) = css_next_child((pos), (parent)))

struct cgroup_subsys_state *
css_next_descendant_pre(struct cgroup_subsys_state *pos,
                        struct cgroup_subsys_state *css);

struct cgroup_subsys_state *
css_rightmost_descendant(struct cgroup_subsys_state *pos);

/**
 * css_for_each_descendant_pre - pre-order walk of a css's descendants
 * @pos: the css * to use as the loop cursor
 * @root: css whose descendants to walk
 *
 * Walk @root's descendants.  @root is included in the iteration and the
 * first node to be visited.  Must be called under rcu_read_lock().  A
 * descendant css which hasn't finished ->css_online() or already has
 * finished ->css_offline() may show up during traversal and it's each
 * subsystem's responsibility to verify that each @pos is alive.
 *
 * If a subsystem synchronizes against the parent in its ->css_online() and
 * before starting iterating, and synchronizes against @pos on each
 * iteration, any descendant css which finished ->css_online() is
 * guaranteed to be visible in the future iterations.
 *
 * In other words, the following guarantees that a descendant can't escape
 * state updates of its ancestors.
 *
 * my_online(@css)
 * {
 *      Lock @css's parent and @css;
 *      Inherit state from the parent;
 *      Unlock both.
 * }
 *
 * my_update_state(@css)
 * {
 *      css_for_each_descendant_pre(@pos, @css) {
 *              Lock @pos;
 *              if (@pos == @css)
 *                      Update @css's state;
 *              else
 *                      Verify @pos is alive and inherit state from its parent;
 *              Unlock @pos;
 *      }
 * }
 *
 * As long as the inheriting step, including checking the parent state, is
 * enclosed inside @pos locking, double-locking the parent isn't necessary
 * while inheriting.  The state update to the parent is guaranteed to be
 * visible by walking order and, as long as inheriting operations to the
 * same @pos are atomic to each other, multiple updates racing each other
 * still result in the correct state.  It's guaranateed that at least one
 * inheritance happens for any css after the latest update to its parent.
 *
 * If checking parent's state requires locking the parent, each inheriting
 * iteration should lock and unlock both @pos->parent and @pos.
 *
 * Alternatively, a subsystem may choose to use a single global lock to
 * synchronize ->css_online() and ->css_offline() against tree-walking
 * operations.
 *
 * It is allowed to temporarily drop RCU read lock during iteration.  The
 * caller is responsible for ensuring that @pos remains accessible until
 * the start of the next iteration by, for example, bumping the css refcnt.
 */
#define css_for_each_descendant_pre(pos, css)                           \
        for ((pos) = css_next_descendant_pre(NULL, (css)); (pos);       \
             (pos) = css_next_descendant_pre((pos), (css)))

struct cgroup_subsys_state *
css_next_descendant_post(struct cgroup_subsys_state *pos,
                         struct cgroup_subsys_state *css);

/**
 * css_for_each_descendant_post - post-order walk of a css's descendants
 * @pos: the css * to use as the loop cursor
 * @css: css whose descendants to walk
 *
 * Similar to css_for_each_descendant_pre() but performs post-order
 * traversal instead.  @root is included in the iteration and the last
 * node to be visited.  Note that the walk visibility guarantee described
 * in pre-order walk doesn't apply the same to post-order walks.
 */
#define css_for_each_descendant_post(pos, css)                          \
        for ((pos) = css_next_descendant_post(NULL, (css)); (pos);      \
             (pos) = css_next_descendant_post((pos), (css)))

/* A css_task_iter should be treated as an opaque object */
struct css_task_iter {
        struct cgroup_subsys_state      *origin_css;
        struct list_head                *cset_link;
        struct list_head                *task;
};

void css_task_iter_start(struct cgroup_subsys_state *css,
                         struct css_task_iter *it);
struct task_struct *css_task_iter_next(struct css_task_iter *it);
void css_task_iter_end(struct css_task_iter *it);

int cgroup_attach_task_all(struct task_struct *from, struct task_struct *);
int cgroup_transfer_tasks(struct cgroup *to, struct cgroup *from);

struct cgroup_subsys_state *css_tryget_from_dir(struct dentry *dentry,
                                                struct cgroup_subsys *ss);

#else /* !CONFIG_CGROUPS */

static inline int cgroup_init_early(void) { return 0; }
static inline int cgroup_init(void) { return 0; }
static inline void cgroup_fork(struct task_struct *p) {}
static inline void cgroup_post_fork(struct task_struct *p) {}
static inline void cgroup_exit(struct task_struct *p, int callbacks) {}

static inline int cgroupstats_build(struct cgroupstats *stats,
                                        struct dentry *dentry)
{
        return -EINVAL;
}

/* No cgroups - nothing to do */
static inline int cgroup_attach_task_all(struct task_struct *from,
                                         struct task_struct *t)
{
        return 0;
}

#endif /* !CONFIG_CGROUPS */

#endif /* _LINUX_CGROUP_H */