Re: [RFC PATCH 1/5] x86: introduce preemption disable prefix

[Andy Lutomirski <luto@kernel.org>]

> On Oct 18, 2018, at 6:08 PM, Nadav Amit <namit@vmware.com> wrote:
>
> at 10:00 AM, Andy Lutomirski <luto@amacapital.net> wrote:
>
>>
>>
>>> On Oct 18, 2018, at 9:47 AM, Nadav Amit <namit@vmware.com> wrote:
>>>
>>> at 8:51 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>>>
>>>>> On Wed, Oct 17, 2018 at 8:12 PM Nadav Amit <namit@vmware.com> wrote:
>>>>> at 6:22 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>>>>>
>>>>>>> On Oct 17, 2018, at 5:54 PM, Nadav Amit <namit@vmware.com> wrote:
>>>>>>>
>>>>>>> It is sometimes beneficial to prevent preemption for very few
>>>>>>> instructions, or prevent preemption for some instructions that precede
>>>>>>> a branch (this latter case will be introduced in the next patches).
>>>>>>>
>>>>>>> To provide such functionality on x86-64, we use an empty REX-prefix
>>>>>>> (opcode 0x40) as an indication that preemption is disabled for the
>>>>>>> following instruction.
>>>>>>
>>>>>> Nifty!
>>>>>>
>>>>>> That being said, I think you have a few bugs. First, you can’t just ignore
>>>>>> a rescheduling interrupt, as you introduce unbounded latency when this
>>>>>> happens — you’re effectively emulating preempt_enable_no_resched(), which
>>>>>> is not a drop-in replacement for preempt_enable(). To fix this, you may
>>>>>> need to jump to a slow-path trampoline that calls schedule() at the end or
>>>>>> consider rewinding one instruction instead. Or use TF, which is only a
>>>>>> little bit terrifying…
>>>>>
>>>>> Yes, I didn’t pay enough attention here. For my use-case, I think that the
>>>>> easiest solution would be to make synchronize_sched() ignore preemptions
>>>>> that happen while the prefix is detected. It would slightly change the
>>>>> meaning of the prefix.
>>>
>>> So thinking about it further, rewinding the instruction seems the easiest
>>> and most robust solution. I’ll do it.
>>>
>>>>>> You also aren’t accounting for the case where you get an exception that
>>>>>> is, in turn, preempted.
>>>>>
>>>>> Hmm.. Can you give me an example for such an exception in my use-case? I
>>>>> cannot think of an exception that might be preempted (assuming #BP, #MC
>>>>> cannot be preempted).
>>>>
>>>> Look for cond_local_irq_enable().
>>>
>>> I looked at it. Yet, I still don’t see how exceptions might happen in my
>>> use-case, but having said that - this can be fixed too.
>>
>> I’m not totally certain there’s a case that matters.  But it’s worth checking
>
> I am still checking. But, I wanted to ask you whether the existing code is
> correct, since it seems to me that others do the same mistake I did, unless
> I don’t understand the code.
>
> Consider for example do_int3(), and see my inlined comments:
>
> dotraplinkage void notrace do_int3(struct pt_regs *regs, long error_code)
> {
>    ...
>    ist_enter(regs);        // => preempt_disable()
>    cond_local_irq_enable(regs);    // => assume it enables IRQs
>
>    ...
>    // resched irq can be delivered here. It will not caused rescheduling
>    // since preemption is disabled
>
>    cond_local_irq_disable(regs);    // => assume it disables IRQs
>    ist_exit(regs);            // => preempt_enable_no_resched()
> }
>
> At this point resched will not happen for unbounded length of time (unless
> there is another point when exiting the trap handler that checks if
> preemption should take place).

I think it's only a bug in the cases where someone uses extable to fix
up an int3 (which would be nuts) or that we oops.  But I should still
fix it.  In the normal case where int3 was in user code, we'll miss
the reschedule in do_trap(), but we'll reschedule in
prepare_exit_to_usermode() -> exit_to_usermode_loop().

>
> Another example is __BPF_PROG_RUN_ARRAY(), which also uses
> preempt_enable_no_resched().

Alexei, I think this code is just wrong. Do you know why it uses
preempt_enable_no_resched()?

Oleg, the code in kernel/signal.c:

                preempt_disable();
                read_unlock(&tasklist_lock);
                preempt_enable_no_resched();
                freezable_schedule();

looks bogus.  I don't get what it's trying to achieve with
preempt_disable(), and I also don't see why no_resched does anything.
Sure, it prevents a reschedule triggered during read_unlock() from
causing a reschedule, but it doesn't prevent an interrupt immediately
after the preempt_enable_no_resched() call from scheduling.

--Andy

>
> Am I missing something?
>
> Thanks,
> Nadav