linux-kernel - Re: [RFC][PATCH] exec: Conceal the other threads from wakeups during exec

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Date:   Fri, 31 Jul 2020 11:50:07 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>, Pavel Machek <pavel@....cz>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>
Subject: Re: [RFC][PATCH] exec: Conceal the other threads from wakeups during exec

Oleg Nesterov <oleg@...hat.com> writes:

> Eric, I won't comment the intent, but I too do not understand this idea.
>
> On 07/30, Eric W. Biederman wrote:
>>
>> [This change requires more work to handle TASK_STOPPED and TASK_TRACED]
>
> Yes. And it is not clear to me how can you solve this.

I was imagining something putting TASK_STOPPED and TASK_TRACED in a loop
that verified they should be in that state before exiting so they could
handle spurious wake ups.

There are a many subtlties in that code, especially in the conversion
fo TASK_STOPPED to TASK_TRACED.  So I suspect something more would be
required but I have not looked yet to see how tricky that would be.

>> [This adds a new lock ordering dependency siglock -> pi_lock -> rq_lock ]
>
> Not really, ttwu() can be safely called with siglock held and it takes
> pi_lock + rq_lock. Say, signal_wake_up().

Good point.

>> +int make_task_wakekill(struct task_struct *p)
>> +{
>> +	unsigned long flags;
>> +	int cpu, success = 0;
>> +	struct rq_flags rf;
>> +	struct rq *rq;
>> +	long state;
>> +
>> +	/* Assumes p != current */
>> +	preempt_disable();
>> +	/*
>> +	 * If we are going to change a thread waiting for CONDITION we
>> +	 * need to ensure that CONDITION=1 done by the caller can not be
>> +	 * reordered with p->state check below. This pairs with mb() in
>> +	 * set_current_state() the waiting thread does.
>> +	 */
>> +	raw_spin_lock_irqsave(&p->pi_lock, flags);
>> +	smp_mb__after_spinlock();
>> +	state = p->state;
>> +
>> +	/* FIXME handle TASK_STOPPED and TASK_TRACED */
>> +	if ((state == TASK_KILLABLE) ||
>> +	    (state == TASK_INTERRUPTIBLE)) {
>> +		success = 1;
>> +		cpu = task_cpu(p);
>> +		rq = cpu_rq(cpu);
>> +		rq_lock(rq, &rf);
>> +		p->state = TASK_WAKEKILL;
>
> You can only do this if the task was already deactivated. Just suppose it
> is preempted or does something like
>
> 	set_current_sate(TASK_INTERRUPTIBLE);
>
> 	if (CONDITION) {
> 		// make_task_wakekill() sets state = TASK_WAKEKILL
> 		__set_current_state(TASK_RUNNING);
> 		return;
> 	}
>
> 	schedule();

You are quite right.

So that bit of code would need to be:
	if (!task->on_rq)
        	goto out;
	if ((state == TASK_KILLABLE) ||
            (state == TASK_INTERRUPTIBLE)) {
            ...

Eric