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

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Date:   Thu, 18 Oct 2018 03:12:07 +0000
From:   Nadav Amit <namit@...are.com>
To:     Andy Lutomirski <luto@...capital.net>
CC:     Ingo Molnar <mingo@...hat.com>, Andy 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>,
        David Woodhouse <dwmw@...zon.co.uk>
Subject: Re: [RFC PATCH 1/5] x86: introduce preemption disable prefix

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).

I agree that for super-general case this might be inappropriate.