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Subject[PATCHSET cgroup] extend threadgroup locking

cgroup currently only blocks new threads from joining the target
threadgroup during migration, and on-going migration could race
against exec and exit leading to interesting problems - the symmetry
between various attach methods, task exiting during method execution,
->exit() racing against attach methods, migrating task switching basic
properties during exec and so on.

This patchset extends threadgroup locking such that it covers all
operations which can alter the threadgroup - fork, exit and exec, and
update cgroup to take advantage of it. rwsem read ops are added to
exit path but exec is excluded by grabbing the existing
cred_guard_mutex from threadgroup locking helper.

This makes threadgroup locking complete and resolves cgroup issues
stemming from the target taskset being unstable.

This patchset is on top of the current pm-freezer + "freezer: fixes &
simplifications" patchset and contains the following four patches.
Patch list and diffstat follow.


[PATCH 1/4] cgroup: change locking order in attach_task_by_pid()
[PATCH 2/4] threadgroup: rename signal->threadgroup_fork_lock to
[PATCH 3/4] threadgroup: extend threadgroup_lock() to cover exit and
[PATCH 4/4] cgroup: always lock threadgroup during migration

include/linux/init_task.h | 9 ++----
include/linux/sched.h | 58 ++++++++++++++++++++++++++++---------------
kernel/cgroup.c | 62 +++++++++++++++++++++-------------------------
kernel/exit.c | 16 ++++++++---
kernel/fork.c | 8 ++---
5 files changed, 88 insertions(+), 65 deletions(-)



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