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

["Luis Claudio R. Goncalves" <lgoncalv@redhat.com>]

On Fri, Dec 19, 2025 at 11:22:02PM +0800, Hao Li wrote:
> On Fri, Dec 19, 2025 at 10:29:11AM -0300, Luis Claudio R. Goncalves wrote:
> > On Fri, Dec 19, 2025 at 10:31:55AM +0100, Vlastimil Babka wrote:
> > > On 12/19/25 09:57, 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.
> > > > 
> > > > 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 also checking preempt_count() on PREEMPT_RT, ensuring
> > > > kmalloc_nolock() returns NULL early when called from any
> > > > non-preemptible context.
> > > > 
> > > > 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>
> > > > ---
> > > > 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..1dd8a25664c5 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) && (in_nmi() || in_hardirq() || preempt_count() ))
> > > 
> > > AFAICS we can just simplify that to preempt_count() then, since in_nmi() and
> > > in_hardirq() both are a special cases of that.
> > > 
> > > Any comment from RT folks please?
> > 
> > Maybe, for the purpose of this change, using in_atomic() or !preemptible()
> > would be a bit more descriptive, as both macros check preempt_count()?
> 
> Hi,
> 
> I might be misunderstanding the situation, but my current understanding
> is as follows:
> 
> __might_sleep will report this BUG if it is called with IRQs disabled or
> in atomic context. Therefore, to avoid this BUG, it seems necessary to
> check preemptible(), since in_atomic() alone does not appear to be
> sufficient.

You are correct. I focused in the condition proposed (for which
preempt_count() was enough) and missed the real requirement.
 
> As a side note, once Vlastimil's "sheaves for all" branch is merged into
> mainline, the local_lock_cpu_slab(s, flags); statement that currently
> triggers the BUG is expected to be removed. Furthermore, the entire
> nolock path in SLUB is planned to be implemented using trylock
> semantics, which should eliminate the possibility of sleeping, even on
> RT kernels. At that point, it seems we would only need to guard against
> deadlock risks from NMI and IRQ, so this condition might need to be
> reverted to in_nmi() || in_hardirq() again.
> 
> Please let me know if I'm missing something here or if there are
> additional constraints I haven't considered. I'd appreciate any
> corrections or further insights.
> 
> Thanks
> 
> > 
> > Luis
> >  
> > > > +		/*
> > > > +		 * kmalloc_nolock() in PREEMPT_RT is not supported from
> > > > +		 * non-preemptible context because local_lock becomes a
> > > > +		 * sleeping lock on RT.
> > > > +		 */
> > > >  		return NULL;
> > > >  retry:
> > > >  	if (unlikely(size > KMALLOC_MAX_CACHE_SIZE))
> > > > 
> > > > base-commit: 559e608c46553c107dbba19dae0854af7b219400
> > > > --
> > > > 2.52.0
> > > > 
> > > 
> > > 
> > ---end quoted text---
> > 
> > 
> 
---end quoted text---