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

[Masami Hiramatsu]

On Fri, 19 Oct 2018 04:44:33 +0000
Nadav Amit <namit@xxxxxxxxxx> wrote:

> at 9:29 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> 
> >> On Oct 18, 2018, at 6:08 PM, Nadav Amit <namit@xxxxxxxxxx> wrote:
> >> 
> >> at 10:00 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> >> 
> >>>> On Oct 18, 2018, at 9:47 AM, Nadav Amit <namit@xxxxxxxxxx> wrote:
> >>>> 
> >>>> at 8:51 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> >>>> 
> >>>>>> On Wed, Oct 17, 2018 at 8:12 PM Nadav Amit <namit@xxxxxxxxxx> wrote:
> >>>>>> at 6:22 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> >>>>>> 
> >>>>>>>> On Oct 17, 2018, at 5:54 PM, Nadav Amit <namit@xxxxxxxxxx> 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().
> 
> Thanks for your quick response, and sorry for bothering instead of dealing
> with it. Note that do_debug() does something similar to do_int3().
> 
> And then there is optimized_callback() that also uses
> preempt_enable_no_resched(). I think the original use was correct, but then
> a19b2e3d7839 ("kprobes/x86: Remove IRQ disabling from ftrace-based/optimized
> kprobes”) removed the IRQ disabling, while leaving
> preempt_enable_no_resched() . No?

Ah, good catch!
Indeed, we don't need to stick on no_resched anymore.

Thanks!


-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>