Re: [PATCH v2 1/2] rust: add abstraction for struct device

[Greg KH <gregkh@linuxfoundation.org>]

On Wed, Jun 12, 2024 at 05:35:21PM +0200, Danilo Krummrich wrote:
> On Wed, Jun 12, 2024 at 05:02:52PM +0200, Greg KH wrote:
> > On Wed, Jun 12, 2024 at 04:51:42PM +0200, Danilo Krummrich wrote:
> > > On 6/11/24 18:13, Boqun Feng wrote:
> > > > On Tue, Jun 11, 2024 at 03:29:22PM +0200, Greg KH wrote:
> > > > > On Tue, Jun 11, 2024 at 03:21:31PM +0200, Danilo Krummrich wrote:
> > > > > > ...hence, I agree we should indeed add to the #Invariants and #Safety section
> > > > > > that `->release` must be callable  from any thread.
> > > > > > 
> > > > > > However, this is just theory, do we actually have cases where `device::release`
> > > > 
> > > > @Danilo, right, it's only theorical, but it's good to call it out since
> > > > it's the requirement for a safe Rust abstraction.
> > > 
> > > Similar to my previous reply, if we want to call this out as safety requirement
> > > in `Device::from_raw`, we probably want to add it to the documentation of the C
> > > `struct device`, such that we can argue that this is an invariant of C's
> > > `struct device`.
> > > 
> > > Otherwise we'd have to write something like:
> > > 
> > > "It must also be ensured that the `->release` function of a `struct device` can
> > > be called from any non-atomic context. While not being officially documented this
> > > is guaranteed by the invariant of `struct device`."
> > 
> > In the 20+ years of the driver model being part of the kernel, I don't
> > think this has come up yet, so maybe you can call the release function
> > in irq context.  I don't know, I was just guessing :)
> 
> Ah, I see. I thought you know and it's defined, but just not documented.
> 
> This means it's simply undefined what we expect to happen when the last
> reference of a device is dropped from atomic context.
> 
> Now, I understand (and would even expect) that practically this has never been
> an issue. You'd need two circumstances, release() actually does something that
> is not allowed in atomic context plus the last device reference is dropped from
> atomic context - rather unlikely.
> 
> > 
> > So let's not go adding constraints that we just do not have please.
> > Same goes for the C code, so the rust code is no different here.
> 
> I agree we shouldn't add random constraints, but for writing safe code we also
> have to rely on defined behavior.

As the rust code is relying on C code that could change at any point in
time, how can that ever be "safe"?  :)

Sorry, this type of definition annoys me.

> I see two options:
> 
> (1) We globally (for struct device) define from which context release() is
>     allowed to be called.

If you want, feel free to do that work please.  And then find out how to
enforce it in the driver core.

> (2) We define it for the Rust abstraction only and just constrain it to
>     non-atomic context to be able to give a safety guarantee. We can't say
>     "might be safe from any context, but we don't know".

Why can't you say that?  Your "saftey" barrier ends/begins at the
interaction with the rust/c layer.  You can't "guarantee" anything on
the C side...

> But again, this is really just a formality, the C code does it all the way and
> practically there never was an issue, which means we actually do follow some
> rules, it's just about writing them down. :)

Again, this has NEVER come up in 20+ years, so maybe it's just not an
issue?  Not to say it isn't, but maybe do some tests and see what
happens...

thanks,

greg k-h