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

[Thomas Gleixner]

On Mon, Sep 14 2020 at 23:39, Linus Torvalds wrote:
> On Mon, Sep 14, 2020 at 11:24 PM Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx> 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