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Date: Tue, 24 Feb 2026 17:35:23 +0100
Message-Id: <DGNC6GEH8EV7.2WWAQ8DNCLRAB@kernel.org>
Subject: Re: [RFC PATCH v3 1/5] rtc: add device selector for rtc_class_ops
 callbacks
Cc: "Rafael J. Wysocki" <rafael@kernel.org>, "Alvin Sun"
 <alvin.sun@linux.dev>, "Miguel Ojeda" <ojeda@kernel.org>, "Boqun Feng"
 <boqun.feng@gmail.com>, "Gary Guo" <gary@garyguo.net>,
 =?utf-8?q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>, "Benno Lossin"
 <lossin@kernel.org>, "Andreas Hindborg" <a.hindborg@kernel.org>, "Alice
 Ryhl" <aliceryhl@google.com>, "Trevor Gross" <tmgross@umich.edu>,
 <linux-rtc@vger.kernel.org>, <rust-for-linux@vger.kernel.org>, "Greg
 Kroah-Hartman" <gregkh@linuxfoundation.org>
To: "Alexandre Belloni" <alexandre.belloni@bootlin.com>
From: "Danilo Krummrich" <dakr@kernel.org>
References: <20260220225341c5eeb835@mail.local>
 <d1c9e33b-e1f3-41c6-af5e-a85fe2b86d10@linux.dev>
 <20260221111619162a41a1@mail.local>
 <CAJZ5v0jo2sLKWVOBJz7QP9x_aMZbaVx+ES7QwYWkTzHp7d2xLQ@mail.gmail.com>
 <DGKPPQI0QE73.S8I1M5NCI2BV@kernel.org> <20260222000556ea1938c0@mail.local>
 <DGLI4H9M0T6D.25RTLDVU5JRBE@kernel.org>
 <CAJZ5v0gtiQxBCknkaOzLKrDqUQfhKh_UjQkvgxJBL4UthbCOkg@mail.gmail.com>
 <DGLMGEZTM7E2.Y8VV9I6LI1P6@kernel.org>
 <DGMR9XOWP1V0.3C9219TYPXV6J@kernel.org> <2026022415010804e28202@mail.local>
In-Reply-To: <2026022415010804e28202@mail.local>

On Tue Feb 24, 2026 at 4:01 PM CET, Alexandre Belloni wrote:
> On 24/02/2026 01:12:32+0100, Danilo Krummrich wrote:
>> 	impl pci::Driver for SampleDriver {
>> 	    fn probe(pdev: &pci::Device<Core>, info: &Self::IdInfo) -> impl Pin=
Init<Self, Error> {
>> 	        let dev =3D pdev.as_ref();
>>=20
>> 	        let rtc_data =3D impl_pin_init!(SampleRtcData {
>> 	            io: iomap_region_sized::<BAR0_SIZE>(0, c"my_rtc/bar0")?,
>> 	            hw_variant: VendorVariant::StV1,
>> 	        });
>>=20
>> 	        let rtc =3D rtc::Device::new(dev, rtc_data)?;
>>=20
>> 	        // Internally calls `devres::register(rtc::Registration::new())=
`.
>> 	        rtc::Registration::register(rtc)?;
>>=20
>> 	        Ok(impl_pin_init!(Self {
>> 	            // Give the IRQ handler a reference count of the `rtc::Devi=
ce`.
>> 	            irq <- irq::Registration::new(..., rtc.clone()),
>> 	            rtc,
>> 	        })
>
> I can't really read rust yet but this seems to open a race condition
> with userspace if irq::Registration::new(...) fails, there is an
> ordering constraint you missed.

(I did not have any specific hardware in mind when sketching this up (e.g. =
an
IRQ could also only be needed in bus device callbacks, e.g. for loading fir=
mware
etc.). But for RTC it obviously is common that it is relevant to the class
device too.)

So, I assume you mean because there could already be an ioctl before the IR=
Q has
been successfully registered, and this ioctl may wait for an IRQ?

In this case the irq::Registration should go into rtc_data instead to accou=
nt
for this dependency. Unfortunately, this is a semantic dependency that we c=
an't
always catch at compile time.

The reason we sometimes can is because, if you would need access to the
irq::Registration from ioctls (e.g. for calling synchronize(), enable(),
disable() etc.) it would be caught, because you couldn't access it without =
it
being in rtc_data in the first place, and being forced to have it in rtc_da=
ta
guarantees that the ordering can't be wrong.