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

[Alexei Starovoitov]

On Thu, Apr 16, 2026 at 8:59 PM Harry Yoo (Oracle) <harry@xxxxxxxxxx> wrote:
>
> On Thu, Apr 16, 2026 at 07:41:02PM -0700, Alexei Starovoitov wrote:
> > On Thu, Apr 16, 2026 at 7:34 PM Harry Yoo (Oracle) <harry@xxxxxxxxxx> 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@xxxxxxxxxx> 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;
>
> Just realized that it's unnecessarily restrictive to disallow
> hardirq context on UP.
>
> Tried below and it resolves the issue.

Ack. Even better :)