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

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Thu, Apr 16, 2026 at 7:35 AM Harry Yoo (Oracle) <harry@kernel.org> wrote:
>
> 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;

This one.

> 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;

Doesn't need to be that drastic.