Merge branch 'for-3.4' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup

Pull cgroup changes from Tejun Heo:
 "Out of the 8 commits, one fixes a long-standing locking issue around
  tasklist walking and others are cleanups."

* 'for-3.4' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup:
  cgroup: Walk task list under tasklist_lock in cgroup_enable_task_cg_list
  cgroup: Remove wrong comment on cgroup_enable_task_cg_list()
  cgroup: remove cgroup_subsys argument from callbacks
  cgroup: remove extra calls to find_existing_css_set
  cgroup: replace tasklist_lock with rcu_read_lock
  cgroup: simplify double-check locking in cgroup_attach_proc
  cgroup: move struct cgroup_pidlist out from the header file
  cgroup: remove cgroup_attach_task_current_cg()

5 files changed, 158 insertions, 229 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index a5d3b5325f77..c6877fe9a831 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -818,7 +818,7 @@ static int cgroup_call_pre_destroy(struct cgroup *cgrp)
 
         for_each_subsys(cgrp->root, ss)
                 if (ss->pre_destroy) {
-                        ret = ss->pre_destroy(ss, cgrp);
+                        ret = ss->pre_destroy(cgrp);
                         if (ret)
                                 break;
                 }
@@ -846,7 +846,7 @@ static void cgroup_diput(struct dentry *dentry, struct inode *inode)
                  * Release the subsystem state objects.
                  */
                 for_each_subsys(cgrp->root, ss)
-                        ss->destroy(ss, cgrp);
+                        ss->destroy(cgrp);
 
                 cgrp->root->number_of_cgroups--;
                 mutex_unlock(&cgroup_mutex);
@@ -1015,7 +1015,7 @@ static int rebind_subsystems(struct cgroupfs_root *root,
                         list_move(&ss->sibling, &root->subsys_list);
                         ss->root = root;
                         if (ss->bind)
-                                ss->bind(ss, cgrp);
+                                ss->bind(cgrp);
                         mutex_unlock(&ss->hierarchy_mutex);
                         /* refcount was already taken, and we're keeping it */
                 } else if (bit & removed_bits) {
@@ -1025,7 +1025,7 @@ static int rebind_subsystems(struct cgroupfs_root *root,
                         BUG_ON(cgrp->subsys[i]->cgroup != cgrp);
                         mutex_lock(&ss->hierarchy_mutex);
                         if (ss->bind)
-                                ss->bind(ss, dummytop);
+                                ss->bind(dummytop);
                         dummytop->subsys[i]->cgroup = dummytop;
                         cgrp->subsys[i] = NULL;
                         subsys[i]->root = &rootnode;
@@ -1763,6 +1763,7 @@ EXPORT_SYMBOL_GPL(cgroup_path);
 struct task_and_cgroup {
         struct task_struct      *task;
         struct cgroup           *cgrp;
+        struct css_set          *cg;
 };
 
 struct cgroup_taskset {
@@ -1843,11 +1844,10 @@ EXPORT_SYMBOL_GPL(cgroup_taskset_size);
  * will already exist. If not set, this function might sleep, and can fail with
  * -ENOMEM. Must be called with cgroup_mutex and threadgroup locked.
  */
-static int cgroup_task_migrate(struct cgroup *cgrp, struct cgroup *oldcgrp,
-                               struct task_struct *tsk, bool guarantee)
+static void cgroup_task_migrate(struct cgroup *cgrp, struct cgroup *oldcgrp,
+                                struct task_struct *tsk, struct css_set *newcg)
 {
         struct css_set *oldcg;
-        struct css_set *newcg;
 
         /*
          * We are synchronized through threadgroup_lock() against PF_EXITING
@@ -1857,23 +1857,6 @@ static int cgroup_task_migrate(struct cgroup *cgrp, struct cgroup *oldcgrp,
         WARN_ON_ONCE(tsk->flags & PF_EXITING);
         oldcg = tsk->cgroups;
 
-        /* locate or allocate a new css_set for this task. */
-        if (guarantee) {
-                /* we know the css_set we want already exists. */
-                struct cgroup_subsys_state *template[CGROUP_SUBSYS_COUNT];
-                read_lock(&css_set_lock);
-                newcg = find_existing_css_set(oldcg, cgrp, template);
-                BUG_ON(!newcg);
-                get_css_set(newcg);
-                read_unlock(&css_set_lock);
-        } else {
-                might_sleep();
-                /* find_css_set will give us newcg already referenced. */
-                newcg = find_css_set(oldcg, cgrp);
-                if (!newcg)
-                        return -ENOMEM;
-        }
-
         task_lock(tsk);
         rcu_assign_pointer(tsk->cgroups, newcg);
         task_unlock(tsk);
@@ -1892,7 +1875,6 @@ static int cgroup_task_migrate(struct cgroup *cgrp, struct cgroup *oldcgrp,
         put_css_set(oldcg);
 
         set_bit(CGRP_RELEASABLE, &oldcgrp->flags);
-        return 0;
 }
 
 /**
@@ -1910,6 +1892,7 @@ int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk)
         struct cgroup *oldcgrp;
         struct cgroupfs_root *root = cgrp->root;
         struct cgroup_taskset tset = { };
+        struct css_set *newcg;
 
         /* @tsk either already exited or can't exit until the end */
         if (tsk->flags & PF_EXITING)
@@ -1925,7 +1908,7 @@ int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk)
 
         for_each_subsys(root, ss) {
                 if (ss->can_attach) {
-                        retval = ss->can_attach(ss, cgrp, &tset);
+                        retval = ss->can_attach(cgrp, &tset);
                         if (retval) {
                                 /*
                                  * Remember on which subsystem the can_attach()
@@ -1939,13 +1922,17 @@ int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk)
                 }
         }
 
-        retval = cgroup_task_migrate(cgrp, oldcgrp, tsk, false);
-        if (retval)
+        newcg = find_css_set(tsk->cgroups, cgrp);
+        if (!newcg) {
+                retval = -ENOMEM;
                 goto out;
+        }
+
+        cgroup_task_migrate(cgrp, oldcgrp, tsk, newcg);
 
         for_each_subsys(root, ss) {
                 if (ss->attach)
-                        ss->attach(ss, cgrp, &tset);
+                        ss->attach(cgrp, &tset);
         }
 
         synchronize_rcu();
@@ -1967,7 +1954,7 @@ out:
                                  */
                                 break;
                         if (ss->cancel_attach)
-                                ss->cancel_attach(ss, cgrp, &tset);
+                                ss->cancel_attach(cgrp, &tset);
                 }
         }
         return retval;
@@ -1997,66 +1984,6 @@ int cgroup_attach_task_all(struct task_struct *from, struct task_struct *tsk)
 }
 EXPORT_SYMBOL_GPL(cgroup_attach_task_all);
 
-/*
- * cgroup_attach_proc works in two stages, the first of which prefetches all
- * new css_sets needed (to make sure we have enough memory before committing
- * to the move) and stores them in a list of entries of the following type.
- * TODO: possible optimization: use css_set->rcu_head for chaining instead
- */
-struct cg_list_entry {
-        struct css_set *cg;
-        struct list_head links;
-};
-
-static bool css_set_check_fetched(struct cgroup *cgrp,
-                                  struct task_struct *tsk, struct css_set *cg,
-                                  struct list_head *newcg_list)
-{
-        struct css_set *newcg;
-        struct cg_list_entry *cg_entry;
-        struct cgroup_subsys_state *template[CGROUP_SUBSYS_COUNT];
-
-        read_lock(&css_set_lock);
-        newcg = find_existing_css_set(cg, cgrp, template);
-        read_unlock(&css_set_lock);
-
-        /* doesn't exist at all? */
-        if (!newcg)
-                return false;
-        /* see if it's already in the list */
-        list_for_each_entry(cg_entry, newcg_list, links)
-                if (cg_entry->cg == newcg)
-                        return true;
-
-        /* not found */
-        return false;
-}
-
-/*
- * Find the new css_set and store it in the list in preparation for moving the
- * given task to the given cgroup. Returns 0 or -ENOMEM.
- */
-static int css_set_prefetch(struct cgroup *cgrp, struct css_set *cg,
-                            struct list_head *newcg_list)
-{
-        struct css_set *newcg;
-        struct cg_list_entry *cg_entry;
-
-        /* ensure a new css_set will exist for this thread */
-        newcg = find_css_set(cg, cgrp);
-        if (!newcg)
-                return -ENOMEM;
-        /* add it to the list */
-        cg_entry = kmalloc(sizeof(struct cg_list_entry), GFP_KERNEL);
-        if (!cg_entry) {
-                put_css_set(newcg);
-                return -ENOMEM;
-        }
-        cg_entry->cg = newcg;
-        list_add(&cg_entry->links, newcg_list);
-        return 0;
-}
-
 /**
  * cgroup_attach_proc - attach all threads in a threadgroup to a cgroup
  * @cgrp: the cgroup to attach to
@@ -2070,20 +1997,12 @@ static int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
         int retval, i, group_size;
         struct cgroup_subsys *ss, *failed_ss = NULL;
         /* guaranteed to be initialized later, but the compiler needs this */
-        struct css_set *oldcg;
         struct cgroupfs_root *root = cgrp->root;
         /* threadgroup list cursor and array */
         struct task_struct *tsk;
         struct task_and_cgroup *tc;
         struct flex_array *group;
         struct cgroup_taskset tset = { };
-        /*
-         * we need to make sure we have css_sets for all the tasks we're
-         * going to move -before- we actually start moving them, so that in
-         * case we get an ENOMEM we can bail out before making any changes.
-         */
-        struct list_head newcg_list;
-        struct cg_list_entry *cg_entry, *temp_nobe;
 
         /*
          * step 0: in order to do expensive, possibly blocking operations for
@@ -2102,23 +2021,14 @@ static int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
         if (retval)
                 goto out_free_group_list;
 
-        /* prevent changes to the threadgroup list while we take a snapshot. */
-        read_lock(&tasklist_lock);
-        if (!thread_group_leader(leader)) {
-                /*
-                 * a race with de_thread from another thread's exec() may strip
-                 * us of our leadership, making while_each_thread unsafe to use
-                 * on this task. if this happens, there is no choice but to
-                 * throw this task away and try again (from cgroup_procs_write);
-                 * this is "double-double-toil-and-trouble-check locking".
-                 */
-                read_unlock(&tasklist_lock);
-                retval = -EAGAIN;
-                goto out_free_group_list;
-        }
-
         tsk = leader;
         i = 0;
+        /*
+         * Prevent freeing of tasks while we take a snapshot. Tasks that are
+         * already PF_EXITING could be freed from underneath us unless we
+         * take an rcu_read_lock.
+         */
+        rcu_read_lock();
         do {
                 struct task_and_cgroup ent;
 
@@ -2128,24 +2038,24 @@ static int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
 
                 /* as per above, nr_threads may decrease, but not increase. */
                 BUG_ON(i >= group_size);
-                /*
-                 * saying GFP_ATOMIC has no effect here because we did prealloc
-                 * earlier, but it's good form to communicate our expectations.
-                 */
                 ent.task = tsk;
                 ent.cgrp = task_cgroup_from_root(tsk, root);
                 /* nothing to do if this task is already in the cgroup */
                 if (ent.cgrp == cgrp)
                         continue;
+                /*
+                 * saying GFP_ATOMIC has no effect here because we did prealloc
+                 * earlier, but it's good form to communicate our expectations.
+                 */
                 retval = flex_array_put(group, i, &ent, GFP_ATOMIC);
                 BUG_ON(retval != 0);
                 i++;
         } while_each_thread(leader, tsk);
+        rcu_read_unlock();
         /* remember the number of threads in the array for later. */
         group_size = i;
         tset.tc_array = group;
         tset.tc_array_len = group_size;
-        read_unlock(&tasklist_lock);
 
         /* methods shouldn't be called if no task is actually migrating */
         retval = 0;
@@ -2157,7 +2067,7 @@ static int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
          */
         for_each_subsys(root, ss) {
                 if (ss->can_attach) {
-                        retval = ss->can_attach(ss, cgrp, &tset);
+                        retval = ss->can_attach(cgrp, &tset);
                         if (retval) {
                                 failed_ss = ss;
                                 goto out_cancel_attach;
@@ -2169,17 +2079,12 @@ static int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
          * step 2: make sure css_sets exist for all threads to be migrated.
          * we use find_css_set, which allocates a new one if necessary.
          */
-        INIT_LIST_HEAD(&newcg_list);
         for (i = 0; i < group_size; i++) {
                 tc = flex_array_get(group, i);
-                oldcg = tc->task->cgroups;
-
-                /* if we don't already have it in the list get a new one */
-                if (!css_set_check_fetched(cgrp, tc->task, oldcg,
-                                           &newcg_list)) {
-                        retval = css_set_prefetch(cgrp, oldcg, &newcg_list);
-                        if (retval)
-                                goto out_list_teardown;
+                tc->cg = find_css_set(tc->task->cgroups, cgrp);
+                if (!tc->cg) {
+                        retval = -ENOMEM;
+                        goto out_put_css_set_refs;
                 }
         }
 
@@ -2190,8 +2095,7 @@ static int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
          */
         for (i = 0; i < group_size; i++) {
                 tc = flex_array_get(group, i);
-                retval = cgroup_task_migrate(cgrp, tc->cgrp, tc->task, true);
-                BUG_ON(retval);
+                cgroup_task_migrate(cgrp, tc->cgrp, tc->task, tc->cg);
         }
         /* nothing is sensitive to fork() after this point. */
 
@@ -2200,7 +2104,7 @@ static int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
          */
         for_each_subsys(root, ss) {
                 if (ss->attach)
-                        ss->attach(ss, cgrp, &tset);
+                        ss->attach(cgrp, &tset);
         }
 
         /*
@@ -2209,21 +2113,22 @@ static int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
         synchronize_rcu();
         cgroup_wakeup_rmdir_waiter(cgrp);
         retval = 0;
-out_list_teardown:
-        /* clean up the list of prefetched css_sets. */
-        list_for_each_entry_safe(cg_entry, temp_nobe, &newcg_list, links) {
-                list_del(&cg_entry->links);
-                put_css_set(cg_entry->cg);
-                kfree(cg_entry);
+out_put_css_set_refs:
+        if (retval) {
+                for (i = 0; i < group_size; i++) {
+                        tc = flex_array_get(group, i);
+                        if (!tc->cg)
+                                break;
+                        put_css_set(tc->cg);
+                }
         }
 out_cancel_attach:
-        /* same deal as in cgroup_attach_task */
         if (retval) {
                 for_each_subsys(root, ss) {
                         if (ss == failed_ss)
                                 break;
                         if (ss->cancel_attach)
-                                ss->cancel_attach(ss, cgrp, &tset);
+                                ss->cancel_attach(cgrp, &tset);
                 }
         }
 out_free_group_list:
@@ -2245,22 +2150,14 @@ static int attach_task_by_pid(struct cgroup *cgrp, u64 pid, bool threadgroup)
         if (!cgroup_lock_live_group(cgrp))
                 return -ENODEV;
 
+retry_find_task:
+        rcu_read_lock();
         if (pid) {
-                rcu_read_lock();
                 tsk = find_task_by_vpid(pid);
                 if (!tsk) {
                         rcu_read_unlock();
-                        cgroup_unlock();
-                        return -ESRCH;
-                }
-                if (threadgroup) {
-                        /*
-                         * RCU protects this access, since tsk was found in the
-                         * tid map. a race with de_thread may cause group_leader
-                         * to stop being the leader, but cgroup_attach_proc will
-                         * detect it later.
-                         */
-                        tsk = tsk->group_leader;
+                        ret= -ESRCH;
+                        goto out_unlock_cgroup;
                 }
                 /*
                  * even if we're attaching all tasks in the thread group, we
@@ -2271,29 +2168,38 @@ static int attach_task_by_pid(struct cgroup *cgrp, u64 pid, bool threadgroup)
                     cred->euid != tcred->uid &&
                     cred->euid != tcred->suid) {
                         rcu_read_unlock();
-                        cgroup_unlock();
-                        return -EACCES;
+                        ret = -EACCES;
+                        goto out_unlock_cgroup;
                 }
-                get_task_struct(tsk);
-                rcu_read_unlock();
-        } else {
-                if (threadgroup)
-                        tsk = current->group_leader;
-                else
-                        tsk = current;
-                get_task_struct(tsk);
-        }
-
-        threadgroup_lock(tsk);
+        } else
+                tsk = current;
 
         if (threadgroup)
+                tsk = tsk->group_leader;
+        get_task_struct(tsk);
+        rcu_read_unlock();
+
+        threadgroup_lock(tsk);
+        if (threadgroup) {
+                if (!thread_group_leader(tsk)) {
+                        /*
+                         * a race with de_thread from another thread's exec()
+                         * may strip us of our leadership, if this happens,
+                         * there is no choice but to throw this task away and
+                         * try again; this is
+                         * "double-double-toil-and-trouble-check locking".
+                         */
+                        threadgroup_unlock(tsk);
+                        put_task_struct(tsk);
+                        goto retry_find_task;
+                }
                 ret = cgroup_attach_proc(cgrp, tsk);
-        else
+        } else
                 ret = cgroup_attach_task(cgrp, tsk);
-
         threadgroup_unlock(tsk);
 
         put_task_struct(tsk);
+out_unlock_cgroup:
         cgroup_unlock();
         return ret;
 }
@@ -2305,16 +2211,7 @@ static int cgroup_tasks_write(struct cgroup *cgrp, struct cftype *cft, u64 pid)
 
 static int cgroup_procs_write(struct cgroup *cgrp, struct cftype *cft, u64 tgid)
 {
-        int ret;
-        do {
-                /*
-                 * attach_proc fails with -EAGAIN if threadgroup leadership
-                 * changes in the middle of the operation, in which case we need
-                 * to find the task_struct for the new leader and start over.
-                 */
-                ret = attach_task_by_pid(cgrp, tgid, true);
-        } while (ret == -EAGAIN);
-        return ret;
+        return attach_task_by_pid(cgrp, tgid, true);
 }
 
 /**
@@ -2804,15 +2701,20 @@ static void cgroup_advance_iter(struct cgroup *cgrp,
  * using their cgroups capability, we don't maintain the lists running
  * through each css_set to its tasks until we see the list actually
  * used - in other words after the first call to cgroup_iter_start().
- *
- * The tasklist_lock is not held here, as do_each_thread() and
- * while_each_thread() are protected by RCU.
  */
 static void cgroup_enable_task_cg_lists(void)
 {
         struct task_struct *p, *g;
         write_lock(&css_set_lock);
         use_task_css_set_links = 1;
+        /*
+         * We need tasklist_lock because RCU is not safe against
+         * while_each_thread(). Besides, a forking task that has passed
+         * cgroup_post_fork() without seeing use_task_css_set_links = 1
+         * is not guaranteed to have its child immediately visible in the
+         * tasklist if we walk through it with RCU.
+         */
+        read_lock(&tasklist_lock);
         do_each_thread(g, p) {
                 task_lock(p);
                 /*
@@ -2824,6 +2726,7 @@ static void cgroup_enable_task_cg_lists(void)
                         list_add(&p->cg_list, &p->cgroups->tasks);
                 task_unlock(p);
         } while_each_thread(g, p);
+        read_unlock(&tasklist_lock);
         write_unlock(&css_set_lock);
 }
 
@@ -3043,6 +2946,38 @@ int cgroup_scan_tasks(struct cgroup_scanner *scan)
  *
  */
 
+/* which pidlist file are we talking about? */
+enum cgroup_filetype {
+        CGROUP_FILE_PROCS,
+        CGROUP_FILE_TASKS,
+};
+
+/*
+ * A pidlist is a list of pids that virtually represents the contents of one
+ * of the cgroup files ("procs" or "tasks"). We keep a list of such pidlists,
+ * a pair (one each for procs, tasks) for each pid namespace that's relevant
+ * to the cgroup.
+ */
+struct cgroup_pidlist {
+        /*
+         * used to find which pidlist is wanted. doesn't change as long as
+         * this particular list stays in the list.
+        */
+        struct { enum cgroup_filetype type; struct pid_namespace *ns; } key;
+        /* array of xids */
+        pid_t *list;
+        /* how many elements the above list has */
+        int length;
+        /* how many files are using the current array */
+        int use_count;
+        /* each of these stored in a list by its cgroup */
+        struct list_head links;
+        /* pointer to the cgroup we belong to, for list removal purposes */
+        struct cgroup *owner;
+        /* protects the other fields */
+        struct rw_semaphore mutex;
+};
+
 /*
  * The following two functions "fix" the issue where there are more pids
  * than kmalloc will give memory for; in such cases, we use vmalloc/vfree.
@@ -3827,7 +3762,7 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
                 set_bit(CGRP_CLONE_CHILDREN, &cgrp->flags);
 
         for_each_subsys(root, ss) {
-                struct cgroup_subsys_state *css = ss->create(ss, cgrp);
+                struct cgroup_subsys_state *css = ss->create(cgrp);
 
                 if (IS_ERR(css)) {
                         err = PTR_ERR(css);
@@ -3841,7 +3776,7 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
                 }
                 /* At error, ->destroy() callback has to free assigned ID. */
                 if (clone_children(parent) && ss->post_clone)
-                        ss->post_clone(ss, cgrp);
+                        ss->post_clone(cgrp);
         }
 
         cgroup_lock_hierarchy(root);
@@ -3875,7 +3810,7 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
 
         for_each_subsys(root, ss) {
                 if (cgrp->subsys[ss->subsys_id])
-                        ss->destroy(ss, cgrp);
+                        ss->destroy(cgrp);
         }
 
         mutex_unlock(&cgroup_mutex);
@@ -4099,7 +4034,7 @@ static void __init cgroup_init_subsys(struct cgroup_subsys *ss)
         /* Create the top cgroup state for this subsystem */
         list_add(&ss->sibling, &rootnode.subsys_list);
         ss->root = &rootnode;
-        css = ss->create(ss, dummytop);
+        css = ss->create(dummytop);
         /* We don't handle early failures gracefully */
         BUG_ON(IS_ERR(css));
         init_cgroup_css(css, ss, dummytop);
@@ -4188,7 +4123,7 @@ int __init_or_module cgroup_load_subsys(struct cgroup_subsys *ss)
          * no ss->create seems to need anything important in the ss struct, so
          * this can happen first (i.e. before the rootnode attachment).
          */
-        css = ss->create(ss, dummytop);
+        css = ss->create(dummytop);
         if (IS_ERR(css)) {
                 /* failure case - need to deassign the subsys[] slot. */
                 subsys[i] = NULL;
@@ -4206,7 +4141,7 @@ int __init_or_module cgroup_load_subsys(struct cgroup_subsys *ss)
                 int ret = cgroup_init_idr(ss, css);
                 if (ret) {
                         dummytop->subsys[ss->subsys_id] = NULL;
-                        ss->destroy(ss, dummytop);
+                        ss->destroy(dummytop);
                         subsys[i] = NULL;
                         mutex_unlock(&cgroup_mutex);
                         return ret;
@@ -4304,7 +4239,7 @@ void cgroup_unload_subsys(struct cgroup_subsys *ss)
          * pointer to find their state. note that this also takes care of
          * freeing the css_id.
          */
-        ss->destroy(ss, dummytop);
+        ss->destroy(dummytop);
         dummytop->subsys[ss->subsys_id] = NULL;
 
         mutex_unlock(&cgroup_mutex);
@@ -4580,7 +4515,7 @@ void cgroup_fork_callbacks(struct task_struct *child)
                 for (i = 0; i < CGROUP_BUILTIN_SUBSYS_COUNT; i++) {
                         struct cgroup_subsys *ss = subsys[i];
                         if (ss->fork)
-                                ss->fork(ss, child);
+                                ss->fork(child);
                 }
         }
 }
@@ -4596,6 +4531,17 @@ void cgroup_fork_callbacks(struct task_struct *child)
  */
 void cgroup_post_fork(struct task_struct *child)
 {
+        /*
+         * use_task_css_set_links is set to 1 before we walk the tasklist
+         * under the tasklist_lock and we read it here after we added the child
+         * to the tasklist under the tasklist_lock as well. If the child wasn't
+         * yet in the tasklist when we walked through it from
+         * cgroup_enable_task_cg_lists(), then use_task_css_set_links value
+         * should be visible now due to the paired locking and barriers implied
+         * by LOCK/UNLOCK: it is written before the tasklist_lock unlock
+         * in cgroup_enable_task_cg_lists() and read here after the tasklist_lock
+         * lock on fork.
+         */
         if (use_task_css_set_links) {
                 write_lock(&css_set_lock);
                 if (list_empty(&child->cg_list)) {
@@ -4682,7 +4628,7 @@ void cgroup_exit(struct task_struct *tsk, int run_callbacks)
                                 struct cgroup *old_cgrp =
                                         rcu_dereference_raw(cg->subsys[i])->cgroup;
                                 struct cgroup *cgrp = task_cgroup(tsk, i);
-                                ss->exit(ss, cgrp, old_cgrp, tsk);
+                                ss->exit(cgrp, old_cgrp, tsk);
                         }
                 }
         }
@@ -5137,8 +5083,7 @@ struct cgroup_subsys_state *cgroup_css_from_dir(struct file *f, int id)
 }
 
 #ifdef CONFIG_CGROUP_DEBUG
-static struct cgroup_subsys_state *debug_create(struct cgroup_subsys *ss,
-                                                   struct cgroup *cont)
+static struct cgroup_subsys_state *debug_create(struct cgroup *cont)
 {
         struct cgroup_subsys_state *css = kzalloc(sizeof(*css), GFP_KERNEL);
 
@@ -5148,7 +5093,7 @@ static struct cgroup_subsys_state *debug_create(struct cgroup_subsys *ss,
         return css;
 }
 
-static void debug_destroy(struct cgroup_subsys *ss, struct cgroup *cont)
+static void debug_destroy(struct cgroup *cont)
 {
         kfree(cont->subsys[debug_subsys_id]);
 }
diff --git a/kernel/cgroup_freezer.c b/kernel/cgroup_freezer.c
index fc0646b78a64..f86e93920b62 100644
--- a/kernel/cgroup_freezer.c
+++ b/kernel/cgroup_freezer.c
@@ -128,8 +128,7 @@ struct cgroup_subsys freezer_subsys;
  *    task->alloc_lock (inside __thaw_task(), prevents race with refrigerator())
  *     sighand->siglock
  */
-static struct cgroup_subsys_state *freezer_create(struct cgroup_subsys *ss,
-                                                  struct cgroup *cgroup)
+static struct cgroup_subsys_state *freezer_create(struct cgroup *cgroup)
 {
         struct freezer *freezer;
 
@@ -142,8 +141,7 @@ static struct cgroup_subsys_state *freezer_create(struct cgroup_subsys *ss,
         return &freezer->css;
 }
 
-static void freezer_destroy(struct cgroup_subsys *ss,
-                            struct cgroup *cgroup)
+static void freezer_destroy(struct cgroup *cgroup)
 {
         struct freezer *freezer = cgroup_freezer(cgroup);
 
@@ -164,8 +162,7 @@ static bool is_task_frozen_enough(struct task_struct *task)
  * a write to that file racing against an attach, and hence the
  * can_attach() result will remain valid until the attach completes.
  */
-static int freezer_can_attach(struct cgroup_subsys *ss,
-                              struct cgroup *new_cgroup,
+static int freezer_can_attach(struct cgroup *new_cgroup,
                               struct cgroup_taskset *tset)
 {
         struct freezer *freezer;
@@ -185,7 +182,7 @@ static int freezer_can_attach(struct cgroup_subsys *ss,
         return 0;
 }
 
-static void freezer_fork(struct cgroup_subsys *ss, struct task_struct *task)
+static void freezer_fork(struct task_struct *task)
 {
         struct freezer *freezer;
 
diff --git a/kernel/cpuset.c b/kernel/cpuset.c
index a09ac2b9a661..5d575836dba6 100644
--- a/kernel/cpuset.c
+++ b/kernel/cpuset.c
@@ -1399,8 +1399,7 @@ static nodemask_t cpuset_attach_nodemask_from;
 static nodemask_t cpuset_attach_nodemask_to;
 
 /* Called by cgroups to determine if a cpuset is usable; cgroup_mutex held */
-static int cpuset_can_attach(struct cgroup_subsys *ss, struct cgroup *cgrp,
-                             struct cgroup_taskset *tset)
+static int cpuset_can_attach(struct cgroup *cgrp, struct cgroup_taskset *tset)
 {
         struct cpuset *cs = cgroup_cs(cgrp);
         struct task_struct *task;
@@ -1436,8 +1435,7 @@ static int cpuset_can_attach(struct cgroup_subsys *ss, struct cgroup *cgrp,
         return 0;
 }
 
-static void cpuset_attach(struct cgroup_subsys *ss, struct cgroup *cgrp,
-                          struct cgroup_taskset *tset)
+static void cpuset_attach(struct cgroup *cgrp, struct cgroup_taskset *tset)
 {
         struct mm_struct *mm;
         struct task_struct *task;
@@ -1833,8 +1831,7 @@ static int cpuset_populate(struct cgroup_subsys *ss, struct cgroup *cont)
  * (and likewise for mems) to the new cgroup. Called with cgroup_mutex
  * held.
  */
-static void cpuset_post_clone(struct cgroup_subsys *ss,
-                              struct cgroup *cgroup)
+static void cpuset_post_clone(struct cgroup *cgroup)
 {
         struct cgroup *parent, *child;
         struct cpuset *cs, *parent_cs;
@@ -1857,13 +1854,10 @@ static void cpuset_post_clone(struct cgroup_subsys *ss,
 
 /*
  *      cpuset_create - create a cpuset
- *      ss:     cpuset cgroup subsystem
  *      cont:   control group that the new cpuset will be part of
  */
 
-static struct cgroup_subsys_state *cpuset_create(
-        struct cgroup_subsys *ss,
-        struct cgroup *cont)
+static struct cgroup_subsys_state *cpuset_create(struct cgroup *cont)
 {
         struct cpuset *cs;
         struct cpuset *parent;
@@ -1902,7 +1896,7 @@ static struct cgroup_subsys_state *cpuset_create(
  * will call async_rebuild_sched_domains().
  */
 
-static void cpuset_destroy(struct cgroup_subsys *ss, struct cgroup *cont)
+static void cpuset_destroy(struct cgroup *cont)
 {
         struct cpuset *cs = cgroup_cs(cont);
 
diff --git a/kernel/events/core.c b/kernel/events/core.c
index c61234b1a988..4b50357914fb 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -7147,8 +7147,7 @@ unlock:
 device_initcall(perf_event_sysfs_init);
 
 #ifdef CONFIG_CGROUP_PERF
-static struct cgroup_subsys_state *perf_cgroup_create(
-        struct cgroup_subsys *ss, struct cgroup *cont)
+static struct cgroup_subsys_state *perf_cgroup_create(struct cgroup *cont)
 {
         struct perf_cgroup *jc;
 
@@ -7165,8 +7164,7 @@ static struct cgroup_subsys_state *perf_cgroup_create(
         return &jc->css;
 }
 
-static void perf_cgroup_destroy(struct cgroup_subsys *ss,
-                                struct cgroup *cont)
+static void perf_cgroup_destroy(struct cgroup *cont)
 {
         struct perf_cgroup *jc;
         jc = container_of(cgroup_subsys_state(cont, perf_subsys_id),
@@ -7182,8 +7180,7 @@ static int __perf_cgroup_move(void *info)
         return 0;
 }
 
-static void perf_cgroup_attach(struct cgroup_subsys *ss, struct cgroup *cgrp,
-                               struct cgroup_taskset *tset)
+static void perf_cgroup_attach(struct cgroup *cgrp, struct cgroup_taskset *tset)
 {
         struct task_struct *task;
 
@@ -7191,8 +7188,8 @@ static void perf_cgroup_attach(struct cgroup_subsys *ss, struct cgroup *cgrp,
                 task_function_call(task, __perf_cgroup_move, task);
 }
 
-static void perf_cgroup_exit(struct cgroup_subsys *ss, struct cgroup *cgrp,
-                struct cgroup *old_cgrp, struct task_struct *task)
+static void perf_cgroup_exit(struct cgroup *cgrp, struct cgroup *old_cgrp,
+                             struct task_struct *task)
 {
         /*
          * cgroup_exit() is called in the copy_process() failure path.
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index d2bd4647586c..a35cb8dbd8c4 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7571,8 +7571,7 @@ static inline struct task_group *cgroup_tg(struct cgroup *cgrp)
                             struct task_group, css);
 }
 
-static struct cgroup_subsys_state *
-cpu_cgroup_create(struct cgroup_subsys *ss, struct cgroup *cgrp)
+static struct cgroup_subsys_state *cpu_cgroup_create(struct cgroup *cgrp)
 {
         struct task_group *tg, *parent;
 
@@ -7589,15 +7588,14 @@ cpu_cgroup_create(struct cgroup_subsys *ss, struct cgroup *cgrp)
         return &tg->css;
 }
 
-static void
-cpu_cgroup_destroy(struct cgroup_subsys *ss, struct cgroup *cgrp)
+static void cpu_cgroup_destroy(struct cgroup *cgrp)
 {
         struct task_group *tg = cgroup_tg(cgrp);
 
         sched_destroy_group(tg);
 }
 
-static int cpu_cgroup_can_attach(struct cgroup_subsys *ss, struct cgroup *cgrp,
+static int cpu_cgroup_can_attach(struct cgroup *cgrp,
                                  struct cgroup_taskset *tset)
 {
         struct task_struct *task;
@@ -7615,7 +7613,7 @@ static int cpu_cgroup_can_attach(struct cgroup_subsys *ss, struct cgroup *cgrp,
         return 0;
 }
 
-static void cpu_cgroup_attach(struct cgroup_subsys *ss, struct cgroup *cgrp,
+static void cpu_cgroup_attach(struct cgroup *cgrp,
                               struct cgroup_taskset *tset)
 {
         struct task_struct *task;
@@ -7625,8 +7623,8 @@ static void cpu_cgroup_attach(struct cgroup_subsys *ss, struct cgroup *cgrp,
 }
 
 static void
-cpu_cgroup_exit(struct cgroup_subsys *ss, struct cgroup *cgrp,
-                struct cgroup *old_cgrp, struct task_struct *task)
+cpu_cgroup_exit(struct cgroup *cgrp, struct cgroup *old_cgrp,
+                struct task_struct *task)
 {
         /*
          * cgroup_exit() is called in the copy_process() failure path.
@@ -7976,8 +7974,7 @@ struct cgroup_subsys cpu_cgroup_subsys = {
  */
 
 /* create a new cpu accounting group */
-static struct cgroup_subsys_state *cpuacct_create(
-        struct cgroup_subsys *ss, struct cgroup *cgrp)
+static struct cgroup_subsys_state *cpuacct_create(struct cgroup *cgrp)
 {
         struct cpuacct *ca;
 
@@ -8007,8 +8004,7 @@ out:
 }
 
 /* destroy an existing cpu accounting group */
-static void
-cpuacct_destroy(struct cgroup_subsys *ss, struct cgroup *cgrp)
+static void cpuacct_destroy(struct cgroup *cgrp)
 {
         struct cpuacct *ca = cgroup_ca(cgrp);