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

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Date:   Mon, 12 Dec 2022 15:14:34 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Johan Hovold <johan+linaro@...nel.org>,
        Marc Zyngier <maz@...nel.org>
Cc:     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,
        Johan Hovold <johan+linaro@...nel.org>
Subject: Re: [PATCH v3 19/19] irqdomain: Switch to per-domain locking

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.

Thanks,

        tglx