netdev - Re: [PATCH net-next v3 1/3] rust: core abstractions for network PHY drivers

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Date: Thu, 12 Oct 2023 00:07:55 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: FUJITA Tomonori <fujita.tomonori@...il.com>
Cc: netdev@...r.kernel.org, rust-for-linux@...r.kernel.org, andrew@...n.ch,
	miguel.ojeda.sandonis@...il.com, greg@...ah.com, tmgross@...ch.edu
Subject: Re: [PATCH net-next v3 1/3] rust: core abstractions for network PHY
 drivers

On Thu, Oct 12, 2023 at 03:44:44PM +0900, FUJITA Tomonori wrote:
> On Wed, 11 Oct 2023 23:34:18 -0700
> Boqun Feng <boqun.feng@...il.com> wrote:
> 
> > On Thu, Oct 12, 2023 at 02:58:24PM +0900, FUJITA Tomonori wrote:
> >> On Wed, 11 Oct 2023 11:29:45 -0700
> >> Boqun Feng <boqun.feng@...il.com> wrote:
> >> 
> >> > On Mon, Oct 09, 2023 at 10:39:10AM +0900, FUJITA Tomonori wrote:
> >> > [...]
> >> >> +impl Device {
> >> >> +    /// Creates a new [`Device`] instance from a raw pointer.
> >> >> +    ///
> >> >> +    /// # Safety
> >> >> +    ///
> >> >> +    /// For the duration of the lifetime 'a, the pointer must be valid for writing and nobody else
> >> >> +    /// may read or write to the `phy_device` object.
> >> >> +    pub unsafe fn from_raw<'a>(ptr: *mut bindings::phy_device) -> &'a mut Self {
> >> >> +        unsafe { &mut *ptr.cast() }
> >> >> +    }
> >> >> +
> >> >> +    /// Gets the id of the PHY.
> >> >> +    pub fn phy_id(&mut self) -> u32 {
> >> > 
> >> > This function doesn't modify the `self`, why does this need to be a
> >> > `&mut self` function? Ditto for a few functions in this impl block.
> >> > 
> >> > It seems you used `&mut self` for all the functions, which looks like
> >> > more design work is required here.
> >> 
> >> Ah, I can drop all the mut here.
> > 
> > It may not be that easy... IIUC, most of the functions in the `impl`
> > block can only be called correctly with phydev->lock held. In other
> > words, their usage requires exclusive accesses. We should somehow
> > express this in the type system, otherwise someone may lose track on
> > this requirement in the future (for example, calling any function
> > without the lock held).
> >
> > A simple type trick comes to me is that
> > 
> > impl Device {
> >     // rename `from_raw` into `assume_locked`
> >     pub unsafe fn assume_locked<'a>(ptr: *mut bindings::phy_device) -> &'a LockedDevice {
> > 	...
> >     }
> > }
> 
> Hmm, the concept of PHYLIB is that a driver never play with a
> lock. From the perspective of PHYLIB, this abstraction is a PHY
> driver. The abstraction should not touch the lock.
> 

Well, usually we want to describe such a constrait/requirement in the
type system, that's part of the Rust bindings, of course, for some
properties it may be hard, so it may be impossible.

> How can someone lose track on this requirement? The abstraction
> creates a Device instance only inside the callbacks.
> 

Right now, yes. The code in the patch only "creates" a Device inside
the callbacks, but the `Device::from_raw` function doesn't mention any
of this requirement, if the design is only called inside the callbacks,
please add something in the function's `# Safety` requirement, since
voliating this may cause memory safety issue.

Type system and unsafe comments are contracts, if one API has a limited
usage by design, people should be able to find it somewhere in the
contracts.

Regards,
Boqun