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

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Message-ID: <20260114160151.a6y9OxQW@linutronix.de>
Date: Wed, 14 Jan 2026 17:01:51 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Waiman Long <llong@...hat.com>, Thomas Gleixner <tglx@...nel.org>,
	Marc Zyngier <maz@...nel.org>,
	Clark Williams <clrkwllms@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
	LKML <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 2026-01-13 15:25:26 [-0800], Alexei Starovoitov wrote:
> On Tue, Jan 13, 2026 at 3:55 AM Sebastian Andrzej Siewior
> <bigeasy@...utronix.de> wrote:
> > 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.
> 
> Are you forgetting about local_lock_is_locked() in __slab_alloc() ?

Yeah but this just checks it. Further down the road there is
local_lock_cpu_slab() for the allocation and there is no try-lock on RT.

> With sheaves the whole thing will be very different.
Yes.

Sebastian