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

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Date:   Thu, 18 Oct 2018 09:54:05 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Nadav Amit <namit@...are.com>, Ingo Molnar <mingo@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, Nadav Amit <nadav.amit@...il.com>,
        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

On Wed, Oct 17, 2018 at 06:22:48PM -0700, Andy Lutomirski 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...

At which point we're very close to in-kernel rseq.