linux-kernel - Re: [PATCH v4 06/19] irqdomain: Drop revmap mutex

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Message-ID: <87r0vshu1y.ffs@tglx>
Date:   Wed, 18 Jan 2023 14:05:29 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Johan Hovold <johan@...nel.org>
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, Hsin-Yi Wang <hsinyi@...omium.org>,
        Mark-PK Tsai <mark-pk.tsai@...iatek.com>
Subject: Re: [PATCH v4 06/19] irqdomain: Drop revmap mutex

On Wed, Jan 18 2023 at 10:22, Johan Hovold wrote:
> On Tue, Jan 17, 2023 at 10:23:20PM +0100, Thomas Gleixner wrote:
>> On Mon, Jan 16 2023 at 14:50, Johan Hovold wrote:
>> > The global irq_domain_mutex is now held in all paths that update the
>> > revmap structures so there is no longer any need for the revmap mutex.
>> 
>> This can also go after the 3rd race fix, but ...
>> 
>> >  static void irq_domain_clear_mapping(struct irq_domain *domain,
>> >  				     irq_hw_number_t hwirq)
>> >  {
>> > +	lockdep_assert_held(&irq_domain_mutex);
>> 
>> these lockdep asserts want to be part of the [dis]association race
>> fixes. They are completely unrelated to the removal of the revmap_mutex.
>
> No, they are very much related to the removal of the revmap_mutex. These
> functions deal with the revmap structures which before this patch were
> clearly only modified with the revmap_mutex held.
>
> The lockdep assert is here to guarantee that my claim that all current
> (and future) paths that end up modifying these structures do so under
> the irq_domain_mutex instead.
>
>> Your race fixes change the locking and you want to ensure that all
>> callers comply right there, no?
>
> I want to make sure that all callers of these function comply, yes.
> That's why the asserts belong in this patch.

You can do this in a two step approach.

    1) Add the new locking and ensure that the lock is held when
       the functions are called

    2) Safely remove the revmap_mutex because you already established
       that revmap is protected by some other means

Thanks,

        tglx