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From:   Alexei Starovoitov <alexei.starovoitov@gmail.com>
Date:   Fri, 19 Oct 2018 07:47:18 -0700
Message-ID: <CAADnVQ+et48gk7+gsJb5Wmp6pGuU3t=Yg5t73sKxPJOdbu8K8Q@mail.gmail.com>
Subject: Re: [RFC PATCH 1/5] x86: introduce preemption disable prefix
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Andy Lutomirski <luto@kernel.org>, Nadav Amit <namit@vmware.com>,
        Oleg Nesterov <oleg@redhat.com>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        Borislav Petkov <bp@alien8.de>,
        David Woodhouse <dwmw@amazon.co.uk>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Roman Gushchin <guro@fb.com>
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On Fri, Oct 19, 2018 at 1:22 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Thu, Oct 18, 2018 at 10:00:53PM -0700, Alexei Starovoitov wrote:
> > >
> > > >
> > > > Another example is __BPF_PROG_RUN_ARRAY(), which also uses
> > > > preempt_enable_no_resched().
> > >
> > > Alexei, I think this code is just wrong.
> >
> > why 'just wrong' ?
>
> Because you lost a preemption point, this is a no-no.
>
> >
> > > Do you know why it uses
> > > preempt_enable_no_resched()?
> >
> > dont recall precisely.
> > we could be preemptable at the point where macro is called.
> > I think the goal of no_resched was to avoid adding scheduling points
> > where they didn't exist before just because a prog ran for few nsec.
> > May be Daniel or Roman remember.
>
> No, you did the exact opposite, where there previously was a preemption,
> you just ate it. The band saw didn't get stopped in time, you loose your
> hand etc..

Let me do few experiments then.
We will fix it up.