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

[Alexei Starovoitov]

On Fri, Oct 19, 2018 at 1:22 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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.