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

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Date:   Thu, 17 Sep 2020 09:38:54 +0300
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        Daniel Vetter <daniel@...ll.ch>,
        Thomas Gleixner <tglx@...utronix.de>,
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        Shuah Khan <shuah@...nel.org>, rcu@...r.kernel.org,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>
Subject: Re: [patch 00/13] preempt: Make preempt count unconditional

On Wed, 16 Sep 2020 at 21:32, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> But something like a driver list walking thing should not be doing
> different things behind peoples back depending on whether they hold
> spinlocks or not. It should either just work regardless, or there
> should be a flag (or special interface) for the "you're being called
> in a crtitical region".
>
> Because dynamically changing behavior really is very confusing.
>

By the same reasoning, I don't think a generic crypto library should
be playing tricks with preemption en/disabling under the hood when
iterating over some data that is all directly accessible via the
linear map on the platforms that most people care about. And using
kmap_atomic() unconditionally achieves exactly that.

As I argued before, the fact that kmap_atomic() can be called from an
atomic context, and the fact that its implementation on HIGHMEM
platforms requires preemption to be disabled until the next kunmap()
are two different things, and I don't agree with your assertion that
the name kmap_atomic() implies the latter semantics. If we can avoid
disabling preemption on HIGHMEM, as Thomas suggests, we surely don't
need it on !HIGHMEM either, and given that kmap_atomic() is preferred
today anyway, we can just merge the two implementations. Are there any
existing debug features that could help us spot [ab]use of things like
raw per-CPU data within kmap_atomic regions?

Re your point about deprecating HIGHMEM: some work is underway on ARM
to implement a 3.75/3.75 GB kernel/user split on recent LPAE capable
hardware (which shouldn't suffer from the performance issues that
plagued the 4/4 split on i686), and so hopefully, there is a path
forward for ARM that does not rely on HIGHMEM as it does today.