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

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Message-ID: <20181019082202.GB3121@hirez.programming.kicks-ass.net>
Date:   Fri, 19 Oct 2018 10:22:02 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Andy Lutomirski <luto@...nel.org>, Nadav Amit <namit@...are.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        "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>, daniel@...earbox.net,
        guro@...com
Subject: Re: [RFC PATCH 1/5] x86: introduce preemption disable prefix

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..