linux-kernel - Re: [patch 00/13] preempt: Make preempt count unconditional

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Message-ID: <87een35woz.fsf@nanos.tec.linutronix.de>
Date:   Tue, 15 Sep 2020 09:24:44 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Herbert Xu <herbert@...dor.apana.org.au>
Cc:     Ard Biesheuvel <ardb@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Valentin Schneider <valentin.schneider@....com>,
        Richard Henderson <rth@...ddle.net>,
        Ivan Kokshaysky <ink@...assic.park.msu.ru>,
        Matt Turner <mattst88@...il.com>,
        alpha <linux-alpha@...r.kernel.org>,
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        linux-um <linux-um@...ts.infradead.org>,
        Brian Cain <bcain@...eaurora.org>,
        linux-hexagon@...r.kernel.org,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
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        Lai Jiangshan <jiangshanlai@...il.com>,
        Shuah Khan <shuah@...nel.org>, rcu@...r.kernel.org
Subject: Re: [patch 00/13] preempt: Make preempt count unconditional

On Mon, Sep 14 2020 at 23:39, Linus Torvalds wrote:
> On Mon, Sep 14, 2020 at 11:24 PM Herbert Xu <herbert@...dor.apana.org.au> wrote:
>> > But another reason I tried to avoid kmap_atomic() is that it disables
>> > preemption unconditionally, even on 64-bit architectures where HIGHMEM
>> > is irrelevant. So using kmap_atomic() here means that the bulk of
>> > WireGuard packet encryption runs with preemption disabled, essentially
>> > for legacy reasons.
>>
>> Agreed.  We should definitely fix that.
>
> Well, honestly, one big reason for that is debugging.
>
> The *semantics* of the kmap_atomic() is in the name - you can't sleep
> in between it and the kunmap_atomic().
>
> On any sane architecture, kmap_atomic() ends up being a no-op from an
> implementation standpoint, and sleeping would work just fine.
>
> But we very much want to make sure that people don't then write code
> that doesn't work on the bad old 32-bit machines where it really needs
> that sequence to be safe from preemption.

Alternatively we just make highmem a bit more expensive by making these
maps preemptible. RT is doing this for a long time and it's not that
horrible.

The approach is to keep track about the number of active maps in a task
and on an eventual context switch save them away in the task struct and
restore them when the task is scheduled back in.

Thanks,

        tglx