linux-kernel - Re: [PATCH V7 03/16] rust: cpu: Add from

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Message-ID: <20250115083311.lgw2eu7hz43p2olu@vireshk-i7>
Date: Wed, 15 Jan 2025 14:03:11 +0530
From: Viresh Kumar <viresh.kumar@...aro.org>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Greg KH <gregkh@...uxfoundation.org>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	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>,
	Trevor Gross <tmgross@...ch.edu>, linux-pm@...r.kernel.org,
	Vincent Guittot <vincent.guittot@...aro.org>,
	linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org
Subject: Re: [PATCH V7 03/16] rust: cpu: Add from_cpu()

On 15-01-25, 09:10, Alice Ryhl wrote:
> On the C side, how do you normally prevent uses of the device after it
> became invalid?

IIUC, the per-cpu variable (cpu_sys_devices) that stores the pointers
to CPU devices is cleared and later calls to get_cpu_device() returns
NULL.

The hot-unplug notifiers are fired for existing users (which have
registered for the notifier, like cpufreq), wherein they can remove
per-cpu sysfs files for example.

But otherwise there is no way in the C code to disallow users of the
CPU device pointer, if it is already fetched before the CPU is
removed. The device pointer's memory doesn't get free here though and
it works.

-- 
viresh