Re: [PATCH] staging: android/lowmemorykiller: Don't grab tasklist_lock

[Oleg Nesterov <oleg@redhat.com>]

On 02/02, Anton Vorontsov wrote:
>
> On Thu, Feb 02, 2012 at 01:54:41PM +0100, Oleg Nesterov wrote:
> >
> > This has the same problem, force_sig() becomes unsafe.
>
> Ouch, I think I finally got it. So, lock_task_sighand() is trying to
> gracefully grab the lock, checking if the sighand is not NULL (which means,
> per __exit_signal(), that the task is halfway into the grave).

Yes.

> Would the following fix work for the sysrq?
>
> - - - -
> From: Anton Vorontsov <anton.vorontsov@linaro.org>
> Subject: [PATCH] sysrq: Fix unsafe operations on tasks
>
> sysrq should grab the tasklist lock, otherwise calling force_sig() is
> not safe, as it might race with exiting task, which ->sighand might be
> set to NULL already.

And there are other reasons for tasklist. It is not clear why
for_each_process() itself is safe. OK, currently (afaics) the
caller disables irqs, but in theory this doesn't imply rcu_lock.

And it can race with fork() and miss the task, although mostly
in theory.

> --- a/drivers/tty/sysrq.c
> +++ b/drivers/tty/sysrq.c
> @@ -322,11 +322,13 @@ static void send_sig_all(int sig)
>  {
>  	struct task_struct *p;
>
> +	read_lock(&tasklist_lock);
>  	for_each_process(p) {
>  		if (p->mm && !is_global_init(p))
>  			/* Not swapper, init nor kernel thread */
>  			force_sig(sig, p);
>  	}
> +	read_unlock(&tasklist_lock);

Agreed, but force_sig() should not be used anyway. It sends the
signal to the thread, we need to kill the process. The main thread
can exit.

> > With or without this patch, sig == NULL is not possible but !mm is not right,
> > there could be other other threads with mm != NULL.
>
> I'm not sure I completely follow. In the current LMK code, we check for !mm
> because otherwise the potential victim is useless for us (i.e. killing it
> will not free much memory anyway).

This is the common mistake. Consider this program:

	void *thread_func(...)
	{
		use_a_lot_of_memory();
	}

	int main(void)
	{
		pthread_create(..., thread_func, ...);
		pthread_exit();
	}

lowmem_shrink() will skip this task because p->mm == NULL.

Oleg.