Re: [PATCH v2] slab: fix kmalloc_nolock() context check for PREEMPT_RT

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

On Tue, Jan 13, 2026 at 10:00 AM Sebastian Andrzej Siewior
<bigeasy@linutronix.de> wrote:
>
> On 2026-01-13 20:36:39 [+0530], Swaraj Gaikwad wrote:
> > On PREEMPT_RT kernels, local_lock becomes a sleeping lock. The current
> > check in kmalloc_nolock() only verifies we're not in NMI or hard IRQ
> > context, but misses the case where preemption is disabled.
>
> The reasoning was different back then.
>
> > When a BPF program runs from a tracepoint with preemption disabled
> > (preempt_count > 0), kmalloc_nolock() proceeds to call
> > local_lock_irqsave() which attempts to acquire a sleeping lock,
> > triggering:
> >
> >   BUG: sleeping function called from invalid context
> >   in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 6128
> >   preempt_count: 2, expected: 0
> >
> > Fix this by checking !preemptible() on PREEMPT_RT, which directly
> > expresses the constraint that we cannot take a sleeping lock when
> > preemption is disabled. This encompasses the previous checks for NMI
> > and hard IRQ contexts while also catching cases where preemption is
> > disabled.
> >
> > Fixes: af92793e52c3 ("slab: Introduce kmalloc_nolock() and kfree_nolock().")
> > Reported-by: syzbot+b1546ad4a95331b2101e@syzkaller.appspotmail.com
> > Closes: https://syzkaller.appspot.com/bug?extid=b1546ad4a95331b2101e
> > Signed-off-by: Swaraj Gaikwad <swarajgaikwad1925@gmail.com>
> > ---
>
> Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
>
> for now.
>
> > Changes in v2:
> > - Simplified condition from (in_nmi() || in_hardirq() || preempt_count())
> >   to !preemptible() as suggested by Luis Claudio R. Goncalves and agreed
> >   by Vlastimil Babka
> > - Updated comment to reflect the more descriptive check
> >
> > Tested by building with syz config and running the syzbot
> > reproducer - kernel no longer crashes.
> >
> >  mm/slub.c | 8 ++++++--
> >  1 file changed, 6 insertions(+), 2 deletions(-)
> >
> > diff --git a/mm/slub.c b/mm/slub.c
> > index 2acce22590f8..642f4744d5c6 100644
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -5689,8 +5689,12 @@ void *kmalloc_nolock_noprof(size_t size, gfp_t gfp_flags, int node)
> >       if (unlikely(!size))
> >               return ZERO_SIZE_PTR;
> >
> > -     if (IS_ENABLED(CONFIG_PREEMPT_RT) && (in_nmi() || in_hardirq()))
> > -             /* kmalloc_nolock() in PREEMPT_RT is not supported from irq */
> > +     if (IS_ENABLED(CONFIG_PREEMPT_RT) && !preemptible())
> > +             /*
> > +              * kmalloc_nolock() in PREEMPT_RT is not supported from
> > +              * non-preemptible context because local_lock becomes a
> > +              * sleeping lock on RT.
>
> I would say that despite the _nolock() suffix a local_lock() is still
> acquired. The !PREEMPT_RT does a trylock.
>
> As I noticed this myself today while looking at other patches, was the
> trylock removed on RT by accident, was it there only in an earlier
> version which was never merged and will it ever come back so we can go
> back to !nmi || !hardirq?

The root cause of this syzbot splat is preempt_disable() in
trace_virtio_transport_alloc_pkt() that is being fixed separately.
I guess this patch doesn't hurt, but I suspect with tracepoints
moving to srcu_fast syzbot won't be able to find
preempt_disable() + kmalloc_nolock() case

Acked-by: Alexei Starovoitov <ast@kernel.org>

for now :)
until shaves come.