linux-kernel - Re: [PATCH -next for tip:x86/pti] x86/tlb: drop unneeded local vars in enable_l1d_flush_for

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Message-ID: <87blhni1pg.fsf@nanos.tec.linutronix.de>
Date:   Wed, 30 Sep 2020 20:00:59 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Lukas Bulwahn <lukas.bulwahn@...il.com>,
        Balbir Singh <sblbir@...zon.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com,
        kernel-janitors@...r.kernel.org, linux-safety@...ts.elisa.tech
Subject: Re: [PATCH -next for tip:x86/pti] x86/tlb: drop unneeded local vars in enable_l1d_flush_for_task()

On Wed, Sep 30 2020 at 19:03, Peter Zijlstra wrote:
> On Wed, Sep 30, 2020 at 05:40:08PM +0200, Thomas Gleixner wrote:
> Also, that preempt_disable() in there doesn't actually do anything.
> Worse, preempt_disable(); for_each_cpu(); is an anti-pattern. It mixes
> static_cpu_has() and boot_cpu_has() in the same bloody condition and has
> a pointless ret variable.

I absolutely agree and I really missed it when looking at it before
merging. cpus_read_lock()/unlock() is the right thing to do if at all.

> It's shoddy code, that only works if you align the planets right. We
> really shouldn't provide interfaces that are this bad.
>
> It's correct operation is only by accident.

True :(

I understand Balbirs problem and it makes some sense to provide a
solution. We can:

    1) reject set_affinity() if the task has that flush muck enabled
       and user space tries to move it to a SMT enabled core

    2) disable the muck if if detects that it is runs on a SMT enabled
       core suddenly (hotplug says hello)

       This one is nasty because there is no feedback to user space
       about the wreckage.

Thanks,

        tglx