Re: [PATCH 1/7] rust: file: add Rust abstraction for `struct file`

[Alice Ryhl]

Matthew Wilcox <willy@xxxxxxxxxxxxx>
> I haven't looked at how Rust-for-Linux handles errors yet, but it's
> disappointing to see that it doesn't do something like the PTR_ERR /
> ERR_PTR / IS_ERR C thing under the hood.

It would be cool to do that, but we haven't written the infrastructure
to do that yet. (Note that in this particular case, the C function also
returns the error as a null pointer.)

>> @@ -157,6 +158,12 @@ void rust_helper_init_work_with_key(struct work_struct *work, work_func_t func,
>>  }
>>  EXPORT_SYMBOL_GPL(rust_helper_init_work_with_key);
>>  
>> +struct file *rust_helper_get_file(struct file *f)
>> +{
>> +	return get_file(f);
>> +}
>> +EXPORT_SYMBOL_GPL(rust_helper_get_file);
> 
> This is ridiculous.  A function call instead of doing the
> atomic_long_inc() in Rust?

I think there are two factors to consider here:

First, doing the atomic increment from Rust currently runs into the
memory model split between the C++ and LKMM memory models. It would be
like using the C11 atomic_fetch_add instead of the one that the Kernel
defines for LKMM using inline assembly. When I discussed this with Paul
McKenney, we were advised that its best to avoid mixing the memory
models.

Avoiding this would require that we replicate the inline assembly that C
uses to define its atomic operations on the Rust side. This is something
that I think should be done, but it hasn't been done yet.


Second, there's potentially an increased maintenance burden when C
methods are reimplemented in Rust. Any change to the implementation on
the C side would have to be reflected on the Rust side.

Alice