[PATCH 0/2] initial while_each_thread() fixes

[Oleg Nesterov <oleg@redhat.com>]

Hello.

This was reported several times, I believe the first report is
http://marc.info/?l=linux-kernel&m=127688978121665. Hmm, 3 years
ago. The lockless while_each_thread() is racy and broken, almost
every user can loop forever.

Recently people started to report they actually hit this problem in
oom_kill.c. This doesn't really matter and I can be wrong, but in
fact I do not think they really hit this race, it is very unlikely.
Another problem with while_each_thread() is that it is very easy
to use it wrongly, and oom_kill.c is the good example.

I came to conclusion that it is practically impossible to send a
single series which fixes all problems, too many different users.

So 1/2 adds the new for_each_thread() interface, and 2/2 fixes oom
kill as an example.

We obviously need a lot more changes like 2/2 before we can kill
while_each_thread() and task_struct->thread_group, but I hope they
will be straighforward. And in fact I hope that task->thread_group
can go away before we change all users of while_each_thread().

David, et al, I din't actually test 2/2, I do not know how. Please
review, although it looks simple.

Oleg.

 include/linux/init_task.h |    2 ++
 include/linux/sched.h     |   12 ++++++++++++
 kernel/exit.c             |    1 +
 kernel/fork.c             |    7 +++++++
 mm/oom_kill.c             |   37 ++++++++++++++++++++-----------------
 5 files changed, 42 insertions(+), 17 deletions(-)