Re: [PATCH 08/10] oom: use send_sig() instead force_sig()

[Oleg Nesterov <oleg@redhat.com>]

On 06/08, KOSAKI Motohiro wrote:
>
> Oleg pointed out oom_kill.c has force_sig() abuse. force_sig() mean
> ignore signal mask. but SIGKILL itself is not maskable.

Yes. And we have other reasons to avoid force_sig(). It should be used
only for synchronous signals.

But,

> @@ -399,7 +399,7 @@ static int __oom_kill_process(struct task_struct *p, struct mem_cgroup *mem)
>  	p->rt.time_slice = HZ;
>  	set_tsk_thread_flag(p, TIF_MEMDIE);
>
> -	force_sig(SIGKILL, p);
> +	send_sig(SIGKILL, p, 1);

This is not right, we need send_sig(SIGKILL, p, 0). Better yet,
send_sig_info(SIGKILL, SEND_SIG_NOINFO). I think send_sig() should
die.

The reason is that si_fromuser() must be true, otherwise we can't kill
the SIGNAL_UNKILLABLE (sub-namespace inits) tasks.

Oh. This reminds me, we really need the trivial (but annoying) cleanups
here. The usage of SEND_SIG_ constants is messy, and they should be
renamed at least.

And in fact, we need the new one which acts like SEND_SIG_FORCED but
si_fromuser(). We do not want to allocate the memory when the caller
is oom_kill or zap_pid_ns_processes().

OK. I'll send the simple patch which adds the new helper with the
comment. send_sigkill() or kernel_kill_task(), or do you see a
better name?

Oleg.