Re: [PATCH -next for tip:x86/pti] x86/tlb: drop unneeded local vars in enable_l1d_flush_for_task()

[Thomas Gleixner <tglx@linutronix.de>]

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