Re: [PATCH V7 03/16] rust: cpu: Add from_cpu()

[Alice Ryhl]

On Wed, Jan 15, 2025 at 8:54 AM Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
>
> On Wed, Jan 15, 2025 at 12:50:50PM +0530, Viresh Kumar wrote:
> > On 14-01-25, 19:44, Greg KH wrote:
> > > > +pub fn from_cpu(cpu: u32) -> Result<&'static Device> {
> > > > +    // SAFETY: The pointer returned by `get_cpu_device()`, if not `NULL`, is a valid pointer to
> > > > +    // a `struct device` and is never freed by the C code.
> > >
> > > I thought it was pointed out that it could be freed when a cpu was
> > > hot-unplugged?  Or is that a different device in the cpu code?  We seem
> > > to have 2 of them and it's not obvious which is which :(
> >
> > I did reply [1] to that earlier. The CPU can get unregistered but the
> > memory for the device is never freed (it is part of struct cpu). Some
> > calls on the CPU device may fail later on (if called for an unregisted
> > dev), but should never crash the kernel.
>
> Ah, but that's not really something that SAFETY should override, right?
>
> Yes, you know your implementation of this will stop using the pointer in
> the hotplug callback before it goes away but that's not documented here.
> And having the device "fail" afterward isn't really ok either as you are
> relying on the driver core to always check for this and I'm not so sure
> that it always does on all codepaths.
>
> But, I'm ok with this for now, as you are just copying the bad C model
> at the moment, but it really feels like a huge foot-gun waiting to go
> off.  Any way to put some more documentation here as in "use this at
> your own risk!"?

On the C side, how do you normally prevent uses of the device after it
became invalid?

Alice