Merge branch 'for-4.2' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup

Pull cgroup updates from Tejun Heo:

 - threadgroup_lock got reorganized so that its users can pick the
   actual locking mechanism to use.  Its only user - cgroups - is
   updated to use a percpu_rwsem instead of per-process rwsem.

   This makes things a bit lighter on hot paths and allows cgroups to
   perform and fail multi-task (a process) migrations atomically.
   Multi-task migrations are used in several places including the
   unified hierarchy.

 - Delegation rule and documentation added to unified hierarchy.  This
   will likely be the last interface update from the cgroup core side
   for unified hierarchy before lifting the devel mask.

 - Some groundwork for the pids controller which is scheduled to be
   merged in the coming devel cycle.

* 'for-4.2' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup:
  cgroup: add delegation section to unified hierarchy documentation
  cgroup: require write perm on common ancestor when moving processes on the default hierarchy
  cgroup: separate out cgroup_procs_write_permission() from __cgroup_procs_write()
  kernfs: make kernfs_get_inode() public
  MAINTAINERS: add a cgroup core co-maintainer
  cgroup: fix uninitialised iterator in for_each_subsys_which
  cgroup: replace explicit ss_mask checking with for_each_subsys_which
  cgroup: use bitmask to filter for_each_subsys
  cgroup: add seq_file forward declaration for struct cftype
  cgroup: simplify threadgroup locking
  sched, cgroup: replace signal_struct->group_rwsem with a global percpu_rwsem
  sched, cgroup: reorganize threadgroup locking
  cgroup: switch to unsigned long for bitmasks
  cgroup: reorganize include/linux/cgroup.h
  cgroup: separate out include/linux/cgroup-defs.h
  cgroup: fix some comment typos

Documentation/cgroups/unified-hierarchy.txt

@@ -17,15 +17,18 @@ CONTENTS
 3. Structural Constraints
   3-1. Top-down
   3-2. No internal tasks
-4. Other Changes
-  4-1. [Un]populated Notification
-  4-2. Other Core Changes
-  4-3. Per-Controller Changes
-    4-3-1. blkio
-    4-3-2. cpuset
-    4-3-3. memory
-5. Planned Changes
-  5-1. CAP for resource control
+4. Delegation
+  4-1. Model of delegation
+  4-2. Common ancestor rule
+5. Other Changes
+  5-1. [Un]populated Notification
+  5-2. Other Core Changes
+  5-3. Per-Controller Changes
+    5-3-1. blkio
+    5-3-2. cpuset
+    5-3-3. memory
+6. Planned Changes
+  6-1. CAP for resource control
 
 
 1. Background
@@ -245,9 +248,72 @@ cgroup must create children and transfer all its tasks to the children
 before enabling controllers in its "cgroup.subtree_control" file.
 
 
-4. Other Changes
+4. Delegation
 
-4-1. [Un]populated Notification
+4-1. Model of delegation
+
+A cgroup can be delegated to a less privileged user by granting write
+access of the directory and its "cgroup.procs" file to the user.  Note
+that the resource control knobs in a given directory concern the
+resources of the parent and thus must not be delegated along with the
+directory.
+
+Once delegated, the user can build sub-hierarchy under the directory,
+organize processes as it sees fit and further distribute the resources
+it got from the parent.  The limits and other settings of all resource
+controllers are hierarchical and regardless of what happens in the
+delegated sub-hierarchy, nothing can escape the resource restrictions
+imposed by the parent.
+
+Currently, cgroup doesn't impose any restrictions on the number of
+cgroups in or nesting depth of a delegated sub-hierarchy; however,
+this may in the future be limited explicitly.
+
+
+4-2. Common ancestor rule
+
+On the unified hierarchy, to write to a "cgroup.procs" file, in
+addition to the usual write permission to the file and uid match, the
+writer must also have write access to the "cgroup.procs" file of the
+common ancestor of the source and destination cgroups.  This prevents
+delegatees from smuggling processes across disjoint sub-hierarchies.
+
+Let's say cgroups C0 and C1 have been delegated to user U0 who created
+C00, C01 under C0 and C10 under C1 as follows.
+
+ ~~~~~~~~~~~~~ - C0 - C00
+ ~ cgroup    ~      \ C01
+ ~ hierarchy ~
+ ~~~~~~~~~~~~~ - C1 - C10
+
+C0 and C1 are separate entities in terms of resource distribution
+regardless of their relative positions in the hierarchy.  The
+resources the processes under C0 are entitled to are controlled by
+C0's ancestors and may be completely different from C1.  It's clear
+that the intention of delegating C0 to U0 is allowing U0 to organize
+the processes under C0 and further control the distribution of C0's
+resources.
+
+On traditional hierarchies, if a task has write access to "tasks" or
+"cgroup.procs" file of a cgroup and its uid agrees with the target, it
+can move the target to the cgroup.  In the above example, U0 will not
+only be able to move processes in each sub-hierarchy but also across
+the two sub-hierarchies, effectively allowing it to violate the
+organizational and resource restrictions implied by the hierarchical
+structure above C0 and C1.
+
+On the unified hierarchy, let's say U0 wants to write the pid of a
+process which has a matching uid and is currently in C10 into
+"C00/cgroup.procs".  U0 obviously has write access to the file and
+migration permission on the process; however, the common ancestor of
+the source cgroup C10 and the destination cgroup C00 is above the
+points of delegation and U0 would not have write access to its
+"cgroup.procs" and thus be denied with -EACCES.
+
+
+5. Other Changes
+
+5-1. [Un]populated Notification
 
 cgroup users often need a way to determine when a cgroup's
 subhierarchy becomes empty so that it can be cleaned up.  cgroup
@@ -289,7 +355,7 @@ supported and the interface files "release_agent" and
 "notify_on_release" do not exist.
 
 
-4-2. Other Core Changes
+5-2. Other Core Changes
 
 - None of the mount options is allowed.
 
@@ -306,14 +372,14 @@ supported and the interface files "release_agent" and
 - The "cgroup.clone_children" file is removed.
 
 
-4-3. Per-Controller Changes
+5-3. Per-Controller Changes
 
-4-3-1. blkio
+5-3-1. blkio
 
 - blk-throttle becomes properly hierarchical.
 
 
-4-3-2. cpuset
+5-3-2. cpuset
 
 - Tasks are kept in empty cpusets after hotplug and take on the masks
   of the nearest non-empty ancestor, instead of being moved to it.
@@ -322,7 +388,7 @@ supported and the interface files "release_agent" and
   masks of the nearest non-empty ancestor.
 
 
-4-3-3. memory
+5-3-3. memory
 
 - use_hierarchy is on by default and the cgroup file for the flag is
   not created.
@@ -407,9 +473,9 @@ supported and the interface files "release_agent" and
   memory.low, memory.high, and memory.max will use the string "max" to
   indicate and set the highest possible value.
 
-5. Planned Changes
+6. Planned Changes
 
-5-1. CAP for resource control
+6-1. CAP for resource control
 
 Unified hierarchy will require one of the capabilities(7), which is
 yet to be decided, for all resource control related knobs.  Process

MAINTAINERS

@@ -2816,6 +2816,7 @@ F:	drivers/connector/
 CONTROL GROUP (CGROUP)
 M:	Tejun Heo <tj@kernel.org>
 M:	Li Zefan <lizefan@huawei.com>
+M:	Johannes Weiner <hannes@cmpxchg.org>
 L:	cgroups@vger.kernel.org
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup.git
 S:	Maintained

fs/kernfs/kernfs-internal.h

@@ -76,7 +76,6 @@ extern struct kmem_cache *kernfs_node_cache;
 /*
  * inode.c
  */
-struct inode *kernfs_get_inode(struct super_block *sb, struct kernfs_node *kn);
 void kernfs_evict_inode(struct inode *inode);
 int kernfs_iop_permission(struct inode *inode, int mask);
 int kernfs_iop_setattr(struct dentry *dentry, struct iattr *iattr);

include/linux/init_task.h

@@ -25,13 +25,6 @@
 extern struct files_struct init_files;
 extern struct fs_struct init_fs;
 
-#ifdef CONFIG_CGROUPS
-#define INIT_GROUP_RWSEM(sig)						\
-	.group_rwsem = __RWSEM_INITIALIZER(sig.group_rwsem),
-#else
-#define INIT_GROUP_RWSEM(sig)
-#endif
-
 #ifdef CONFIG_CPUSETS
 #define INIT_CPUSET_SEQ(tsk)							\
 	.mems_allowed_seq = SEQCNT_ZERO(tsk.mems_allowed_seq),
@@ -55,7 +48,6 @@ extern struct fs_struct init_fs;
 	},								\
 	.cred_guard_mutex =						\
 		 __MUTEX_INITIALIZER(sig.cred_guard_mutex),		\
-	INIT_GROUP_RWSEM(sig)						\
 }
 
 extern struct nsproxy init_nsproxy;

include/linux/kernfs.h

@@ -277,6 +277,7 @@ void kernfs_put(struct kernfs_node *kn);
 
 struct kernfs_node *kernfs_node_from_dentry(struct dentry *dentry);
 struct kernfs_root *kernfs_root_from_sb(struct super_block *sb);
+struct inode *kernfs_get_inode(struct super_block *sb, struct kernfs_node *kn);
 
 struct kernfs_root *kernfs_create_root(struct kernfs_syscall_ops *scops,
 				       unsigned int flags, void *priv);
@@ -352,6 +353,10 @@ static inline struct kernfs_node *kernfs_node_from_dentry(struct dentry *dentry)
 static inline struct kernfs_root *kernfs_root_from_sb(struct super_block *sb)
 { return NULL; }
 
+static inline struct inode *
+kernfs_get_inode(struct super_block *sb, struct kernfs_node *kn)
+{ return NULL; }
+
 static inline struct kernfs_root *
 kernfs_create_root(struct kernfs_syscall_ops *scops, unsigned int flags,
 		   void *priv)

include/linux/sched.h

@@ -58,6 +58,7 @@ struct sched_param {
 #include <linux/uidgid.h>
 #include <linux/gfp.h>
 #include <linux/magic.h>
+#include <linux/cgroup-defs.h>
 
 #include <asm/processor.h>
 
@@ -755,18 +756,6 @@ struct signal_struct {
 	unsigned audit_tty_log_passwd;
 	struct tty_audit_buf *tty_audit_buf;
 #endif
-#ifdef CONFIG_CGROUPS
-	/*
-	 * group_rwsem prevents new tasks from entering the threadgroup and
-	 * member tasks from exiting,a more specifically, setting of
-	 * PF_EXITING.  fork and exit paths are protected with this rwsem
-	 * using threadgroup_change_begin/end().  Users which require
-	 * threadgroup to remain stable should use threadgroup_[un]lock()
-	 * which also takes care of exec path.  Currently, cgroup is the
-	 * only user.
-	 */
-	struct rw_semaphore group_rwsem;
-#endif
 
 	oom_flags_t oom_flags;
 	short oom_score_adj;		/* OOM kill score adjustment */
@@ -2725,53 +2714,33 @@ static inline void unlock_task_sighand(struct task_struct *tsk,
 	spin_unlock_irqrestore(&tsk->sighand->siglock, *flags);
 }
 
-#ifdef CONFIG_CGROUPS
-static inline void threadgroup_change_begin(struct task_struct *tsk)
-{
-	down_read(&tsk->signal->group_rwsem);
-}
-static inline void threadgroup_change_end(struct task_struct *tsk)
-{
-	up_read(&tsk->signal->group_rwsem);
-}
-
 /**
- * threadgroup_lock - lock threadgroup
- * @tsk: member task of the threadgroup to lock
- *
- * Lock the threadgroup @tsk belongs to.  No new task is allowed to enter
- * and member tasks aren't allowed to exit (as indicated by PF_EXITING) or
- * change ->group_leader/pid.  This is useful for cases where the threadgroup
- * needs to stay stable across blockable operations.
- *
- * fork and exit paths explicitly call threadgroup_change_{begin|end}() for
- * synchronization.  While held, no new task will be added to threadgroup
- * and no existing live task will have its PF_EXITING set.
+ * threadgroup_change_begin - mark the beginning of changes to a threadgroup
+ * @tsk: task causing the changes
  *
- * de_thread() does threadgroup_change_{begin|end}() when a non-leader
- * sub-thread becomes a new leader.
+ * All operations which modify a threadgroup - a new thread joining the
+ * group, death of a member thread (the assertion of PF_EXITING) and
+ * exec(2) dethreading the process and replacing the leader - are wrapped
+ * by threadgroup_change_{begin|end}().  This is to provide a place which
+ * subsystems needing threadgroup stability can hook into for
+ * synchronization.
  */
-static inline void threadgroup_lock(struct task_struct *tsk)
+static inline void threadgroup_change_begin(struct task_struct *tsk)
 {
-	down_write(&tsk->signal->group_rwsem);
+	might_sleep();
+	cgroup_threadgroup_change_begin(tsk);
 }
 
 /**
- * threadgroup_unlock - unlock threadgroup
- * @tsk: member task of the threadgroup to unlock
+ * threadgroup_change_end - mark the end of changes to a threadgroup
+ * @tsk: task causing the changes
  *
- * Reverse threadgroup_lock().
+ * See threadgroup_change_begin().
  */
-static inline void threadgroup_unlock(struct task_struct *tsk)
+static inline void threadgroup_change_end(struct task_struct *tsk)
 {
-	up_write(&tsk->signal->group_rwsem);
+	cgroup_threadgroup_change_end(tsk);
 }
-#else
-static inline void threadgroup_change_begin(struct task_struct *tsk) {}
-static inline void threadgroup_change_end(struct task_struct *tsk) {}
-static inline void threadgroup_lock(struct task_struct *tsk) {}
-static inline void threadgroup_unlock(struct task_struct *tsk) {}
-#endif
 
 #ifndef __HAVE_THREAD_FUNCTIONS
 

init/Kconfig

@@ -924,6 +924,7 @@ config NUMA_BALANCING_DEFAULT_ENABLED
 menuconfig CGROUPS
 	bool "Control Group support"
 	select KERNFS
+	select PERCPU_RWSEM
 	help
 	  This option adds support for grouping sets of processes together, for
 	  use with process control subsystems such as Cpusets, CFS, memory

kernel/fork.c

@@ -1141,10 +1141,6 @@ static int copy_signal(unsigned long clone_flags, struct task_struct *tsk)
 	tty_audit_fork(sig);
 	sched_autogroup_fork(sig);
 
-#ifdef CONFIG_CGROUPS
-	init_rwsem(&sig->group_rwsem);
-#endif
-
 	sig->oom_score_adj = current->signal->oom_score_adj;
 	sig->oom_score_adj_min = current->signal->oom_score_adj_min;