cgroups: make procs file writable

Make procs file writable to move all threads by tgid at once.

Add functionality that enables users to move all threads in a threadgroup
at once to a cgroup by writing the tgid to the 'cgroup.procs' file.  This
current implementation makes use of a per-threadgroup rwsem that's taken
for reading in the fork() path to prevent newly forking threads within the
threadgroup from "escaping" while the move is in progress.

Signed-off-by: Ben Blum <[email protected]>
Cc: "Eric W. Biederman" <[email protected]>
Cc: Li Zefan <[email protected]>
Cc: Matt Helsley <[email protected]>
Reviewed-by: Paul Menage <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Miao Xie <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Documentation/cgroups/cgroups.txt

@@ -236,7 +236,8 @@ containing the following files describing that cgroup:
  - cgroup.procs: list of tgids in the cgroup.  This list is not
    guaranteed to be sorted or free of duplicate tgids, and userspace
    should sort/uniquify the list if this property is required.
-   This is a read-only file, for now.
+   Writing a thread group id into this file moves all threads in that
+   group into this cgroup.
  - notify_on_release flag: run the release agent on exit?
  - release_agent: the path to use for release notifications (this file
    exists in the top cgroup only)
@@ -430,6 +431,12 @@ You can attach the current shell task by echoing 0:
 
 # echo 0 > tasks
 
+You can use the cgroup.procs file instead of the tasks file to move all
+threads in a threadgroup at once. Echoing the pid of any task in a
+threadgroup to cgroup.procs causes all tasks in that threadgroup to be
+be attached to the cgroup. Writing 0 to cgroup.procs moves all tasks
+in the writing task's threadgroup.
+
 Note: Since every task is always a member of exactly one cgroup in each
 mounted hierarchy, to remove a task from its current cgroup you must
 move it into a new cgroup (possibly the root cgroup) by writing to the