Re: [PATCH 2/4] ARM: Disable HIGHPTE on PREEMPT_RT kernels

Linus Walleij
2024-12-13T00:27:12Z

On Wed, Dec 11, 2024 at 4:22 PM Sebastian Andrzej Siewior
<bigeasy@linutronix.de> wrote:
> On 2024-12-11 14:29:29 [+0100], Linus Walleij wrote:
> > So fast GUP is supposed to be lockless, and should just not
> > have this problem. So it can't be addressing gup_fast_pgd_range()
> > right?
> …
> > I'm more asking if HIGHPTE even acquires a spinlock anymore
> > as it is supposed to be "fast"/lockless. If it does, it is clearly violating
> > the "fast" promise of the fast GUP API and should not exist.
>
> This is lockless on x86. The problem is ARM's
> arch_kmap_local_high_get(). This is where the lock is from.

Aha that calls down to kmap_high_get() that that issues
lock_kmap_any(flags).

But is it really sound that the "fast" API does this? It feels
like a violation of the whole design of the fast stuff.

Yours,
Linus Walleij