Re: + shm-fix-a-race-between-shm_exit-and-shm_init.patch added to -mm tree

[Oleg Nesterov <oleg@redhat.com>]

> From: Vasiliy Kulikov <segoon@openwall.com>
>
> On thread exit shm_exit_ns() is called, it uses shm_ids(ns).rw_mutex.  It
> is initialized in shm_init(), but it is not called yet at the moment of
> kernel threads exit.  Some kernel threads are created in
> do_pre_smp_initcalls(), and shm_init() is called in do_initcalls().
>
> Static initialization of shm_ids(init_ipc_ns).rw_mutex fixes the race.

Yes, it is safe to call down_right() now.

But the code does

	down_write(rw_mutex);
	if (.in_use)
		idr_for_each(.ipcs_idr);

and thus it relies on the static initializer anyway. it is not safe
to do idr_for_each() before idr_init() in theory.

And since we rely on .in_use == 0, why we can't move this check
outside of down_write/up_right to a) optimize the code and b)
fix the problem?

Oleg.