linux-kernel - Re: [PATCH] irqchip/gic-v3-its: Don't acquire rt_spin_lock in allocate_vpe_l1

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Message-ID: <20260113115515.98Mc29Nm@linutronix.de>
Date: Tue, 13 Jan 2026 12:55:15 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Waiman Long <llong@...hat.com>
Cc: Thomas Gleixner <tglx@...nel.org>, Marc Zyngier <maz@...nel.org>,
	Clark Williams <clrkwllms@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-rt-devel@...ts.linux.dev, Vlastimil Babka <vbabka@...e.cz>,
	Alexei Starovoitov <ast@...nel.org>
Subject: Re: [PATCH] irqchip/gic-v3-its: Don't acquire rt_spin_lock in
 allocate_vpe_l1_table()

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