linux-kernel - Re: [PATCH v3 1/5] rust: miscdevice: Provide accessor to pull out miscdevice::this

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Message-ID: <20241206083117.GO8882@google.com>
Date: Fri, 6 Dec 2024 08:31:17 +0000
From: Lee Jones <lee@...nel.org>
To: Boqun Feng <boqun.feng@...il.com>
Cc: Greg KH <gregkh@...uxfoundation.org>, linux-kernel@...r.kernel.org,
	arnd@...db.de, ojeda@...nel.org, alex.gaynor@...il.com,
	gary@...yguo.net, bjorn3_gh@...tonmail.com, benno.lossin@...ton.me,
	a.hindborg@...nel.org, aliceryhl@...gle.com, tmgross@...ch.edu,
	rust-for-linux@...r.kernel.org
Subject: Re: [PATCH v3 1/5] rust: miscdevice: Provide accessor to pull out
 miscdevice::this_device

On Fri, 06 Dec 2024, Boqun Feng wrote:

> On Fri, Dec 06, 2024 at 08:07:51AM +0000, Lee Jones wrote:
> > On Fri, 06 Dec 2024, Boqun Feng wrote:
> > 
> > > On Fri, Dec 06, 2024 at 07:33:09AM +0000, Lee Jones wrote:
> > > > On Fri, 06 Dec 2024, Lee Jones wrote:
> > > > 
> > > > > On Fri, 06 Dec 2024, Greg KH wrote:
> > > > > 
> > > > > > On Thu, Dec 05, 2024 at 04:25:18PM +0000, Lee Jones wrote:
> > > > > > > There are situations where a pointer to a `struct device` will become
> > > > > > > necessary (e.g. for calling into dev_*() functions).  This accessor
> > > > > > > allows callers to pull this out from the `struct miscdevice`.
> > > > > > > 
> > > > > > > Signed-off-by: Lee Jones <lee@...nel.org>
> > > > > > > ---
> > > > > > >  rust/kernel/miscdevice.rs | 9 +++++++++
> > > > > > >  1 file changed, 9 insertions(+)
> > > > > > > 
> > > > > > > diff --git a/rust/kernel/miscdevice.rs b/rust/kernel/miscdevice.rs
> > > > > > > index 7e2a79b3ae26..55340f316006 100644
> > > > > > > --- a/rust/kernel/miscdevice.rs
> > > > > > > +++ b/rust/kernel/miscdevice.rs
> > > > > > > @@ -10,11 +10,13 @@
> > > > > > >  
> > > > > > >  use crate::{
> > > > > > >      bindings,
> > > > > > > +    device::Device,
> > > > > > >      error::{to_result, Error, Result, VTABLE_DEFAULT_ERROR},
> > > > > > >      prelude::*,
> > > > > > >      str::CStr,
> > > > > > >      types::{ForeignOwnable, Opaque},
> > > > > > >  };
> > > > > > > +
> > > > > > >  use core::{
> > > > > > >      ffi::{c_int, c_long, c_uint, c_ulong},
> > > > > > >      marker::PhantomData,
> > > > > > > @@ -84,6 +86,13 @@ pub fn register(opts: MiscDeviceOptions) -> impl PinInit<Self, Error> {
> > > > > > >      pub fn as_raw(&self) -> *mut bindings::miscdevice {
> > > > > > >          self.inner.get()
> > > > > > >      }
> > > > > > > +
> > > > > > > +    /// Returns a pointer to the current Device
> > > > > > > +    pub fn device(&self) -> &Device {
> > > > > > > +        // SAFETY: This is only accessible after a successful register() which always
> > > > > > > +        // initialises this_device with a valid device.
> > > > > > > +        unsafe { Device::as_ref((*self.as_raw()).this_device) }
> > > > > > 
> > > > > > A "raw" pointer that you can do something with without incrementing the
> > > > > > reference count of it?  Oh wait, no, it's the rust device structure.
> > > > > > If so, why isn't this calling the get_device() interface instead?  That
> > > > > > way it's properly incremented and decremented when it "leaves the scope"
> > > > > > right?
> > > > > > 
> > > > > > Or am I missing something here as to why that wouldn't work and this is
> > > > > > the only way to get access to the 'struct device' of this miscdevice?
> > > > > 
> > > > > Fair point.  I'll speak to Alice.
> > > > 
> > > > Alice isn't available yet, so I may be talking out of turn at this
> > > > point, but I just found this is the Device documentation:
> > > > 
> > > >   /// A `Device` instance represents a valid `struct device` created by the C portion of the kernel.
> > > >   ///
> > > >   /// Instances of this type are always reference-counted, that is, a call to `get_device` ensures
> > > >   /// that the allocation remains valid at least until the matching call to `put_device`.
> > > > 
> > > > And:
> > > > 
> > > >   // SAFETY: Instances of `Device` are always reference-counted.
> > > > 
> > > > Ready for some analysis from this beginner?
> > > > 
> > > > Since this impl for Device is AlwaysRefCounted, when any references are
> > > > taken i.e. in the Device::as_ref line above, inc_ref() is implicitly
> > > > called to increase the refcount.  The same will be true of dec_ref()
> > > 
> > > No, inc_ref() is not called implicitly in Device::as_ref().
> > > 
> > > The thing that might "keep" the original `miscdevice::Device` alive is
> > > the lifetime, since the returned `device::Device` reference has the
> > > same life at the input parameter `miscdevice::Device` reference (i.e.
> > > `&self`), so the returned reference cannot outlive `&self`. That means
> > > if compilers find `&self` go out of scope while the returned reference
> > > be still alive, it will report an error.
> > 
> > Okay, so is there something I need to do to ensure we increase the
> > refcount?  Does inc_ref() need calling manually?
> > 
> 
> When you convert a `&Device` into a `ARef<Device>`, Device::inc_ref()
> will be called. You can do that with:
> 
> 	ARef::from(Device::as_ref((*self.as_raw()).this_device))
> 
> You will also need to change the return type. And when an `ARef<Device>`
> goes out of scope, dec_ref() will be called. 

I have been reliably assured by Alice that we don't need to refcount here.

> I had an old patch for a bit document on this part:
> 
> 	https://lore.kernel.org/rust-for-linux/20240710032447.2161189-1-boqun.feng@gmail.com/
> 
> maybe I should send a re-spin.

Very nice!  Yeah, it would be a shame for all that work to go to waste.

-- 
Lee Jones [李琼斯]