Re: [RFC] making nested spin_trylock() work on UP?

[Harry Yoo (Oracle)]

On Thu, Apr 16, 2026 at 07:26:36AM -0700, Alexei Starovoitov wrote:
> On Thu Apr 16, 2026 at 3:05 AM PDT, Vlastimil Babka (SUSE) wrote:
> >> I think we need a special spinlock type that wraps something like this
> >> and use them when spinlocks can be trylock'd in an unknown context:
> >> pcp lock, zone lock, per-node partial slab list lock,
> >> per-node barn lock, etc.
> >
> > Soudns like a lot of hassle for a niche config (SMP=n) where nobody would
> > use e.g. bpf tracing anyway. We already have this in kmalloc_nolock():
> >
> >         /*
> >          * See the comment for the same check in
> >          * alloc_frozen_pages_nolock_noprof()
> >          */
> >         if (IS_ENABLED(CONFIG_PREEMPT_RT) && (in_nmi() || in_hardirq()))
> >                 return NULL;
> >
> > It would be trivial to extend this to !SMP. However it wouldn't cover the
> > kprobe context. Any idea Alexei?

I think Vlastimil meant it'd be trivial to do:

if ((IS_ENABLED(CONFIG_PREEMPT_RT) || !IS_ENABLED(CONFIG_SMP))
		&& (in_nmi() || in_hardirq()))
	return NULL;

But it doesn't cover the case where kprobe hooks an arbitrary function
(in the middle of kmalloc() or kfree()) and calls kmalloc_nolock()?

> Yeah. Totally fine with that.

So I'm confused exactly what you're fine with. Did you mean this?

if (!IS_ENABLED(CONFIG_SMP) ||
		(IS_ENABLED(CONFIG_PREEMPT_RT) && (in_nmi() || in_hardirq())))
       return NULL;

Or am I missing something?

-- 
Cheers,
Harry / Hyeonggon