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

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Date:   Wed, 17 Oct 2018 20:51:22 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Nadav Amit <namit@...are.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Andrew Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        "Woodhouse, David" <dwmw@...zon.co.uk>
Subject: Re: [RFC PATCH 1/5] x86: introduce preemption disable prefix

On Wed, Oct 17, 2018 at 8:12 PM Nadav Amit <namit@...are.com> wrote:
>
> at 6:22 PM, Andy Lutomirski <luto@...capital.net> wrote:
>
> >
> >> On Oct 17, 2018, at 5:54 PM, Nadav Amit <namit@...are.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.
>
> > 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().

--Andy