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

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Date:   Wed, 30 Sep 2020 20:35:52 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
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 08:00:59PM +0200, Thomas Gleixner wrote:
> 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.

Also, I forgot to add, it accesses ->cpus_mask without the proper
locking, so it could be reading intermediate state from whatever cpumask
operation that's in progress.

> 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.

That's and, right, not or. because 1) deals with sched_setffinity()
and 2) deals wit hotplug.

Now 1) requires an arch hook in sched_setaffinity(), something I'm not
keen on providing, once we provide it, who knows what strange and
wonderful things archs will dream up.

And 2) also happens on hot-un-plug, when the task's affinity gets
forced because it became empty. No user feedback there either, and
information is lost.


I suppose we can do 2) but send a signal. That would cover all cases and
keep it in arch code. But yes, that's pretty terrible too.