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

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Message-ID: <87ecnwij44.ffs@tglx>
Date: Sun, 11 Jan 2026 10:39:07 +0100
From: Thomas Gleixner <tglx@...nel.org>
To: Marc Zyngier <maz@...nel.org>
Cc: Waiman Long <longman@...hat.com>, Sebastian Andrzej Siewior
 <bigeasy@...utronix.de>, 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
Subject: Re: [PATCH] irqchip/gic-v3-its: Don't acquire rt_spin_lock in
 allocate_vpe_l1_table()

On Fri, Jan 09 2026 at 16:13, Marc Zyngier wrote:
> On Thu, 08 Jan 2026 22:11:33 +0000,
> Thomas Gleixner <tglx@...nel.org> wrote:
>> At the point where a CPU is brought up, the topology should be known
>> already, which means this can be allocated on the control CPU _before_
>> the new CPU comes up, no?
>
> No. Each CPU finds *itself* in the forest of redistributors, and from
> there tries to find whether it has some shared resource with a CPU
> that has booted before it. That's because firmware is absolutely awful
> and can't present a consistent view of the system.

Groan....

> Anyway, I expect it could be solved by moving this part of the init to
> an ONLINE HP callback.

Which needs to be before CPUHP_AP_IRQ_AFFINITY_ONLINE, but even that
might be to late because there are callbacks in the STARTING section,
i.e. timer, perf, which might rely on interrupts being accessible.

Also that patch seems to be incomplete because there is another
allocation further down in allocate_vpe_l1_table()....

Thanks,

        tglx