Re: [PATCH] irqchip/gic-v3-its: Don't acquire rt_spin_lock in allocate_vpe_l1_table()

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2026-01-12 12:14:30 [-0500], Waiman Long wrote:
> On 1/12/26 10:09 AM, Thomas Gleixner wrote:
> > They might be acquired though. Only alloc_pages_nolock() guarantees that
> > no lock is taken IIRC.
> 
> Thanks for the suggestion. I will look into using that for page allocation.
> I had actually attempt to use kmalloc_nolock() to replace kzalloc()
> initially. Even though it removed the call to rmqueue(), but there were
> other spinlocks in the slub code that were still being acquired like the
> local_lock() or the spinlock in the get_random() code. So I gave up using
> that. Anyway, kmalloc_nolock() doesn't seem to be fully working yet.

with kmalloc_nolock() you have to be able to deal with a NULL pointer.
Looking at kmalloc_nolock(), it has this (in_nmi() || in_hardirq())
check on PREEMPT_RT. The reasoning was unconditional raw_spinlock_t
locking and bad lock-owner recording for hardirq.
There was a trylock path for local_lock to make it work from atomic
context. But from what I can tell this goes
  kmalloc_nolock_noprof() -> __slab_alloc_node() -> __slab_alloc() ->
  ___slab_alloc() -> local_lock_cpu_slab()

The last one does local_lock_irqsave() on PREEMPT_RT which does a
spin_lock(). That means atomic context is not possible. Where did I make
a wrong turn? Or did this change recently? I do remember that Alexei
reworked parts of the allocator to make the local_lock based trylock
allocation work.

> Cheers,
> Longman

Sebastian