linux-kernel - Re: [RFC kgr on klp 9/9] livepatch: send a fake signal to all tasks

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Message-ID: <20150504143423.GA25809@redhat.com>
Date:	Mon, 4 May 2015 16:34:23 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Jiri Slaby <jslaby@...e.cz>
Cc:	live-patching@...r.kernel.org, jpoimboe@...hat.com,
	sjenning@...hat.com, jkosina@...e.cz, vojtech@...e.cz,
	mingo@...hat.com, linux-kernel@...r.kernel.org,
	Miroslav Benes <mbenes@...e.cz>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC kgr on klp 9/9] livepatch: send a fake signal to all tasks

Well, I can't really comment this change because I didn't see other
changes, and I do not know what klp_kgraft_task_in_progress() means...

On 05/04, Jiri Slaby wrote:
>
> Luckily we can force the task to do that by sending it a fake signal,

But note that signal_wake_up(0) won't wake the stopped/traced tasks up.

> +static void klp_kgraft_send_fake_signal(void)
> +{
> +	struct task_struct *p;
> +	unsigned long flags;
> +
> +	read_lock(&tasklist_lock);
> +	for_each_process(p) {

Only the group leader can be klp_kgraft_task_in_progress?

Looks like you need for_each_process_thread()...

> +		/*
> +		 * send fake signal to all non-kthread processes which are still
> +		 * not migrated
> +		 */
> +		if (!(p->flags & PF_KTHREAD) &&

So this can miss the execing kernel thread, I do not know if this is
correct or not. PF_KTHREAD is cleared in flush_old_exec().

> +		    klp_kgraft_task_in_progress(p) &&
> +		    lock_task_sighand(p, &flags)) {

No need for lock_task_sighand(). Just spin_lock_irq(p->sighand->siglock).
tasklist_lock + for_each_process guarantees that "p" has a valid ->sighand.

> +			signal_wake_up(p, 0);

To remind, this won't wakeup a TASK_STOPPED/TRACED thread.

>  void recalc_sigpending(void)
>  {
> -	if (!recalc_sigpending_tsk(current) && !freezing(current))
> +	if (!recalc_sigpending_tsk(current) && !freezing(current) &&
> +	    !klp_kgraft_task_in_progress(current))
>  		clear_thread_flag(TIF_SIGPENDING);

It is not clear from this patch when TIF_SIGPENDING will be cleared.

I assume other changes add some hooks into do_notify_resume/get_signal
paths, otherwise a klp_kgraft_task_in_progress() will spin until
klp_kgraft_task_in_progress(current) becomes "false".

Oleg.

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