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

[Matthew Wilcox <willy@infradead.org>]

On Fri, Dec 13, 2024 at 09:11:57AM +0000, Russell King (Oracle) wrote:
> On Fri, Dec 13, 2024 at 01:27:00AM +0100, Linus Walleij wrote:
> > 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.
> 
> If there's no way to do it lockless, then it has to take the lock.

Well, no, it's allowed to fail.  It could trylock and fail if it can't
get the lock.