linux-kernel - Re: [PATCH] rust: add this

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Message-Id: <20230131160728.400481-1-yakoyoku@gmail.com>
Date:   Tue, 31 Jan 2023 13:07:28 -0300
From:   Martin Rodriguez Reboredo <yakoyoku@...il.com>
To:     gregkh@...uxfoundation.org
Cc:     alex.gaynor@...il.com, bjorn3_gh@...tonmail.com,
        boqun.feng@...il.com, gary@...yguo.net,
        linux-kernel@...r.kernel.org, ojeda@...nel.org,
        rust-for-linux@...r.kernel.org, wedsonaf@...il.com,
        yakoyoku@...il.com
Subject: Re: [PATCH] rust: add this_module macro

On Tue, Jan 31, 2023 at 04:15:51PM +0100, Greg KH wrote:
>On Tue, Jan 31, 2023 at 12:07:45PM -0300, Martin Rodriguez Reboredo wrote:
>> On Tue, Jan 31, 2023 at 02:42:08PM +0100, Greg KH wrote:
>> >On Tue, Jan 31, 2023 at 10:08:41AM -0300, Martin Rodriguez Reboredo wrote:
>> >> Adds a Rust equivalent to the handy THIS_MODULE macro from C.
>> >> 
>> >> Signed-off-by: Martin Rodriguez Reboredo <yakoyoku@...il.com>
>> >> ---
>> >>  rust/kernel/lib.rs | 12 ++++++++++++
>> >>  1 file changed, 12 insertions(+)
>> >> 
>> >> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
>> >> index e0b0e953907d..afb6b0390426 100644
>> >> --- a/rust/kernel/lib.rs
>> >> +++ b/rust/kernel/lib.rs
>> >> @@ -80,6 +80,18 @@ impl ThisModule {
>> >>      }
>> >>  }
>> >>  
>> >> +/// Returns the current module.
>> >> +#[macro_export]
>> >> +macro_rules! this_module {
>> >> +    () => {
>> >> +        if cfg!(MODULE) {
>> >> +            Some(unsafe { $crate::ThisModule::from_ptr(&mut $crate::bindings::__this_module) })
>> >> +        } else {
>> >> +            None
>> >> +        }
>> >> +    };
>> >> +}
>> >
>> >While this is handy, what exactly will it be used for?  The C
>> >wrappers/shim/whatever should probably handle this for you already when
>> >you save this pointer into a structure right?
>> >
>> >Surely you aren't trying to increment your own module's reference count,
>> >right?  That just doesn't work :)
>> >
>> >thanks,
>> >
>> >greg k-h
>> 
>> This was meant for setting the owner field of a file_operations struct
>> or the cra_owner field of crypto_alg and many other structs.
>
>But shouldn't the macro kernel::declare_file_operations() do this for
>you automagically?  You should never have to manually say "this module!"
>to any structure or function call if we do things right.
>
>Yes, many "old school" structures in the kernel do this, but we have
>learned from the 1990's, see the fun wrappers around simple things like
>usb_register_driver(); as an example of how the driver author themselves
>should never see a module pointer anywhere.
>
>> I know that increfing a module without a good reason is dead dumb, so
>> I'm not trying to send things in a downwards spiral. @@@
>
>That's good, but let's not add housekeeping requirements when we do not
>have to do so if at all possible please.
>
>thanks,
>
>greg k-h

*kicks can*, at least I can take some ideas out of this, anyways, thanks
for your reviews.