Re: [RFC PATCH 0/3] gpu: nova-core: add basic timer subdevice implementation

[Boqun Feng <boqun.feng@gmail.com>]

On Thu, Feb 27, 2025 at 03:23:21PM -0400, Jason Gunthorpe wrote:
> On Thu, Feb 27, 2025 at 06:32:15PM +0100, Danilo Krummrich wrote:
> > On Thu, Feb 27, 2025 at 08:55:09AM -0800, Boqun Feng wrote:
> > > On Thu, Feb 27, 2025 at 12:17:33PM -0400, Jason Gunthorpe wrote:
> > > 
> > > > I still wonder why you couldn't also have these reliable reference
> > > > counts rooted on the device driver instead of only on the module.
> > > > 
> > > 
> > > You could put reliable reference counts anywhere you want, as long as it
> > > reflects the resource dependencies.
> > 
> > Right, as I explained in a different reply, the signature for PCI driver probe()
> > looks like this:
> > 
> > 	fn probe(pdev: &mut pci::Device, _info: &Self::IdInfo) -> Result<Pin<KBox<Self>>>
> > 
> > The returned Pin<KBox<Self>> has the lifetime of the driver being bound to the
> > device.
> > 
> > Which means a driver can bind things to this lifetime. But, it isn't forced to,
> > it can also put things into an Arc and share it with the rest of the world.
> 
> This statement right here seems to be the fundamental problem.
> 
> The design pattern says that 'share it with the rest of the world' is
> a bug. A driver following the pattern cannot do that, it must contain
> the driver objects within the driver scope and free them. In C we

I cannot speak for Danilo, but IIUC, the 'share it with the rest of the
world' things are the ones that drivers can share, for example, I
suppose (not a network expert) a NIC driver can share the packet object
with the upper layer of netowrk.

> inspect for this manually, and check for it with kmemleak

In Rust, it's better (of course, depending on your PoV ;-)), because
your driver or module data structures need to track the things they use
(otherwise they will be cancelled and maybe freed, e.g. the hrtimer
case). So you have that part covered by compiler. But could there be
corner cases? Probably. We will just resolve that case by case.

> progamatically.
> 
> It appears to me that the main issue here is that nobody has figured
> out how to make rust have rules that can enforce that design pattern.
> 

Most of the cases, it should be naturally achieved, because you already
bind the objects into your module or driver, otherwise they would be
already cancelled and freed. Handwavingly, it provides a
"data/type-oriented" resource management instead of "oh I have to
remember to call this function before module unload". Again, I believe
there are and will be corner cases, but happy to look into them.

> Have the compiler prevent the driver author from incorrectly extending
> the lifetime of a driver-object beyond the driver's inherent scope, ie
> that Self object above.
> 

Compilers can help in the cases where they know which objects are belong
to a driver/module.

So I think in Rust you can have the "design pattern", the difference is
instead of putting cancel/free functions carefully in some remove()
function, you will need to (still!) carefully arrange the fields in your
driver/module data structure, and you can have more fine grained control
by writting the drop() function for the driver/module data structure.

> Instead we get this:
> 
> > If something is crucial to be bound to the lifetime of a driver being bound to a
> > device (i.e. device resources), you have to expose it as Devres<T>.
> 

I feel I'm still missing some contexts why Devres<T> is related to the
"design pattern", so I will just skip this part for now... Hope we are
on the same page of the "design pattern" in Rust?

Regards,
Boqun

> Which creates a costly way to work around this missing design pattern
> by adding runtime checks to every single access of T in all the
> operational threads. Failable rcu_lock across every batch of register
> access.
> 
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