linux-kernel - Re: [PATCH RFC 02/18] rust: drm: Add Device and Driver abstractions

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Message-ID: <ZAt9X8qDUHQ/YnW8@boqun-archlinux>
Date:   Fri, 10 Mar 2023 10:56:31 -0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     Asahi Lina <lina@...hilina.net>
Cc:     Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Wedson Almeida Filho <wedsonaf@...il.com>,
        Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Christian König <christian.koenig@....com>,
        Luben Tuikov <luben.tuikov@....com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Alyssa Rosenzweig <alyssa@...enzweig.io>,
        Karol Herbst <kherbst@...hat.com>,
        Ella Stanforth <ella@...unix.org>,
        Faith Ekstrand <faith.ekstrand@...labora.com>,
        Mary <mary@...y.zone>, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, rust-for-linux@...r.kernel.org,
        linux-media@...r.kernel.org, linaro-mm-sig@...ts.linaro.org,
        linux-sgx@...r.kernel.org, asahi@...ts.linux.dev
Subject: Re: [PATCH RFC 02/18] rust: drm: Add Device and Driver abstractions

On Tue, Mar 07, 2023 at 11:25:27PM +0900, Asahi Lina wrote:
[...]
> +
> +// SAFETY: `Device` only holds a pointer to a C device, which is safe to be used from any thread.
> +unsafe impl<T: drm::drv::Driver> Send for Device<T> {}
> +
> +// SAFETY: `Device` only holds a pointer to a C device, references to which are safe to be used
> +// from any thread.
> +unsafe impl<T: drm::drv::Driver> Sync for Device<T> {}
> +

Here is the mind model I use to check whether a type is `Send` or
`Sync`

*	If an object of a type can be created on one thread and dropped
	on the another thread, then it's `Send`.

*	If multiple threads can call the immutable functions (i.e.
	functions with `&self`) of the same object of a type, then the
	it's `Sync`.

Maybe it's incomplete, but at least I find it useful to determine
whether a type is `Send` or `Sync`: it's not just the struct
representation, the behaviors (functions) of the struct also matter.

If that looks reasonable to you, maybe update the "SAFETY" comments in
the future version? Thanks ;-)

(I know you brought this up in the meeting, sorry I guess I wasn't fully
woken when answering you ;-))

Regards,
Boqun

> +// Make drm::Device work for dev_info!() and friends
> +unsafe impl<T: drm::drv::Driver> device::RawDevice for Device<T> {
> +    fn raw_device(&self) -> *mut bindings::device {
> +        // SAFETY: ptr must be valid per the type invariant
> +        unsafe { (*self.ptr).dev }
> +    }
> +}
[...]