linux-kernel - Re: [PATCH V6 04/15] rust: device: Add few helpers

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Message-ID: <20250109063605.gmykltd6bnjaspwm@vireshk-i7>
Date: Thu, 9 Jan 2025 12:06:05 +0530
From: Viresh Kumar <viresh.kumar@...aro.org>
To: Danilo Krummrich <dakr@...hat.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
	Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>,
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <benno.lossin@...ton.me>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
	Danilo Krummrich <dakr@...nel.org>, linux-pm@...r.kernel.org,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Stephen Boyd <sboyd@...nel.org>, Nishanth Menon <nm@...com>,
	rust-for-linux@...r.kernel.org,
	Manos Pitsidianakis <manos.pitsidianakis@...aro.org>,
	Erik Schilling <erik.schilling@...aro.org>,
	Alex Bennée <alex.bennee@...aro.org>,
	Joakim Bech <joakim.bech@...aro.org>, Rob Herring <robh@...nel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH V6 04/15] rust: device: Add few helpers

On 08-01-25, 14:42, Danilo Krummrich wrote:
> I think that's really what is needed, otherwise it'll never work with the
> guarantees the Rust `Device` abstraction provides.

> The patch below is still not valid I think. It assumes that a CPU device never
> becomes invalid, but that isn't true.
> 
> There's a hotplug path [1] where the device is unregistered.

Right, though the device pointer always points to valid memory as the struct cpu
is never freed. Isn't that enough for a pointer passed over FFI ? All the
from_cpu() method returns is a reference, which will be used for a short period
only (yes it is about the possibility of something going wrong in that period
only and we need to ensure it doesn't break in such corner cases).

FWIW, the cpufreq framework is registered with CPU hotplug layer, and so
whenever a CPU disappears, the cpufreq core will stop using its device pointer
before the CPU is removed. So technically we shouldn't land in a situation where
the CPU is unregistered and cpufreq core is still using the CPU's device
pointer.

-- 
viresh