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

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Message-ID: <87cy3eg94y.ffs@tglx>
Date: Mon, 12 Jan 2026 16:09:49 +0100
From: Thomas Gleixner <tglx@...nel.org>
To: Waiman Long <llong@...hat.com>, Marc Zyngier <maz@...nel.org>
Cc: 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 Sun, Jan 11 2026 at 18:02, Waiman Long wrote:
> On 1/11/26 5:38 AM, Marc Zyngier wrote:
>>> Also that patch seems to be incomplete because there is another
>>> allocation further down in allocate_vpe_l1_table()....
>> Yeah, I wondered why page allocation wasn't affected by this issue,
>> but didn't try to find out.
>
> The use of GFP_ATOMIC flag in the page allocation request may help it to 
> dip into the reserved area and avoid taking any spinlock. In my own 
> test, just removing the kzalloc() call is enough to avoid any invalid 
> context warning. In the page allocation code, there is a zone lock and a 
> per_cpu_pages lock. They were not acquired in my particular test case, 
> though further investigation may be needed to make sure it is really safe.

They might be acquired though. Only alloc_pages_nolock() guarantees that
no lock is taken IIRC.