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

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Message-ID: <0c33bdee-6de8-4d9f-92ca-4f72c1b6fb9f@suse.cz>
Date: Wed, 14 Jan 2026 18:59:11 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 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, Alexei Starovoitov <ast@...nel.org>
Subject: Re: [PATCH] irqchip/gic-v3-its: Don't acquire rt_spin_lock in
 allocate_vpe_l1_table()

On 1/13/26 12:55, Sebastian Andrzej Siewior wrote:
> 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

Hmm if get_random() code takes a spinlock, we have an unsolved
incompatibility with kmalloc_nolock() and CONFIG_SLAB_FREELIST_RANDOM.

>> 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.

Yes. So even after we fix the current problems with incompatible context, I
think kmalloc_nolock() would still be a bad fit for hw bringup code that
should not really fail. Because the possibility of failure will always
exist. The BPF use case that motivated it is quite different.

> 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