linux-kernel - Re: [patch 2/2] x86/kvm: Sanitize kvm_async_pf_task

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Date:   Sat, 07 Mar 2020 20:18:30 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>,
        Andy Lutomirski <luto@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>, KVM <kvm@...r.kernel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>
Subject: Re: [patch 2/2] x86/kvm: Sanitize kvm_async_pf_task_wait()

Andy Lutomirski <luto@...nel.org> writes:
> On Sat, Mar 7, 2020 at 7:10 AM Andy Lutomirski <luto@...nel.org> wrote:
>> On Sat, Mar 7, 2020 at 2:01 AM Thomas Gleixner <tglx@...utronix.de> wrote:
>> >
>> > Andy Lutomirski <luto@...nel.org> writes:
>
>> Now I'm confused again.  Your patch is very careful not to schedule if
>> we're in an RCU read-side critical section, but the regular preemption
>> code (preempt_schedule_irq, etc) seems to be willing to schedule
>> inside an RCU read-side critical section.  Why is the latter okay but
>> not the async pf case?
>
> I read more docs.  I guess the relevant situation is
> CONFIG_PREEMPT_CPU, in which case it is legal to preempt an RCU
> read-side critical section and obviously legal to put the whole CPU to
> sleep, but it's illegal to explicitly block in an RCU read-side
> critical section.  So I have a question for Paul: is it, in fact,
> entirely illegal to block or merely illegal to block for an
> excessively long time, e.g. waiting for user space or network traffic?

Two issues here:

    - excessive blocking time

    - entering idle with an RCU read side critical section blocking

>  In this situation, we cannot make progress until the host says we
> can, so we are, in effect, blocking until the host tells us to stop
> blocking.  Regardless, I agree that turning IRQs on is reasonable, and
> allowing those IRQs to preempt us is reasonable.
>
> As it stands in your patch, the situation is rather odd: we'll run
> another task if that task *preempts* us (e.g. we block long enough to
> run out of our time slice), but we won't run another task if we aren't
> preempted.  This seems bizarre.

Yes, it looks odd. We could do:

	preempt_disable();
	while (!page_arrived()) {
		if (preempt_count() == 1 && this_cpu_runnable_tasks() > 1) {
        		set_need_resched();
                	schedule_preempt_disabled();
		} else {
                	native_safe_halt();
                        local_irq_disable();
		}
	}
        preempt_enable();

Don't know if it's worth the trouble. But that's not the problem :)

> I think this issue still stands and is actually a fairly easy race to hit.
>
> STI
> IRQ happens and we get preempted
> another task runs and gets the #PF "async pf wakeup" event
> reschedule, back to original task
> HLT

See the other mail about STI :)

Thanks,

        tglx