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

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

On Thu, Apr 16, 2026 at 7:34 PM Harry Yoo (Oracle) <harry@kernel.org> wrote:
>
> On Thu, Apr 16, 2026 at 07:37:49AM -0700, Alexei Starovoitov wrote:
> > 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.
>
> Thanks for clarifying. You mean not covering the kprobe context is fine.
>
> But I have to ask; how is that fine? Wouldn't this leave a small
> possibility for a kmalloc_nolock() caller to trigger
> e.g.) use-after-free bug even without noticing? (yeah, very unlikely
> for somebody to trigger in practice, but not impossible)
>
> If it's unlikely to use bpf tracing on UP anyway, it'd be safer to just
> disallow that to happen to begin with.

Don't fix what is not broken :)
I'm sure there are millions of other issues with UP,
so there is little to zero chance that anyone can repro such a scenario.