linux-kernel - Re: [PATCH v3 19/19] irqdomain: Switch to per-domain locking

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Date:   Mon, 12 Dec 2022 15:29:35 +0100
From:   Johan Hovold <johan@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Johan Hovold <johan+linaro@...nel.org>,
        Marc Zyngier <maz@...nel.org>, x86@...nel.org,
        platform-driver-x86@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-mips@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 19/19] irqdomain: Switch to per-domain locking

On Mon, Dec 12, 2022 at 03:14:34PM +0100, Thomas Gleixner wrote:
> On Fri, Dec 09 2022 at 15:01, Johan Hovold wrote:
> > The IRQ domain structures are currently protected by the global
> > irq_domain_mutex. Switch to using more fine-grained per-domain locking,
> > which may potentially speed up parallel probing somewhat.
> >
> > Note that the domain lock of the root domain (innermost domain) must be
> > used for hierarchical domains. For non-hierarchical domain (as for root
> > domains), the new root pointer is set to the domain itself so that
> > domain->root->mutex can be used in shared code paths.
> >
> > Also note that hierarchical domains should be constructed using
> > irq_domain_create_hierarchy() (or irq_domain_add_hierarchy()) to avoid
> > poking at irqdomain internals.
> 
> While I agree in principle, this change really makes me nervous.
> 
> Any fail in setting up domain->root correctly, e.g. by not using
> irq_domain_create_hierarchy(), cannot be detected and creates nasty to
> debug race conditions.
> 
> So we need some debug mechanism which allows to validate that
> domain->root is set up correctly when domain->parent != NULL.

Lockdep will catch that due to the

	lockdep_assert_held(&domain->root->mutex);

I added to irq_domain_set_mapping() and which is is called for each
(inner) domain in a hierarchy when allocating an IRQ.

Johan