Re: [PATCH RFC 0/2] rust: sync: create lock class using `#[track_caller]`

["Gary Guo" <gary@garyguo.net>]

On Fri Jul 3, 2026 at 3:01 PM BST, Peter Zijlstra wrote:
> On Fri, Jul 03, 2026 at 02:47:25PM +0100, Gary Guo wrote:
>> Currently we're adding more and more macros so that lock classes can be
>> created. The only purpose served by these macros are to create names and
>> static lock classes so they can be tracked by lockdep.
>> 
>> I've come up with an approach that uses `Location::caller` and Rust's
>> `#[track_caller]` feature for this purpose instead. `Location::caller()`
>> will return a `&'static Location<'static>` that lives in `.rodata`. As
>> only addresses matter for static objects, the address of the `Location`
>> themselves could be used for lock classes. The `Location` is of 2 * usize +
>> 8 bytes, so they could host 2 lock class keys, so `DelayedWork` which needs
>> two lock classes can also use this mechanism.
>> 
>> The `Location::caller` could also be used to provide the name; currently,
>> `optional_name` just uses `file_name:line_number` as the name, and this
>> information is available in `Location`. However, `Location` encodes them
>> using file name as a C string plus two `u32`'s for line and column number,
>> so it cannot provide a single name. I came up with an alternative method,
>> which is to use a special string, which lockdep can recognize and delegate
>> back to Rust to print it.
>
> Perhaps add some sample output so us simple folk that still think rust
> looks like line noise can have an idea of what it'll look like. Because
> I just cannot make much sense of the above.

The above is mostly about the Rust implementation detail. On the lockdep side,
essentially the name will be "(rust)", while the actual name can be retrieved
from the lock class key.

So for

    let foo = KBox::pin_init(Mutex::new(42));

in, say, foo.c:123

it will eventually call

    struct rust_location *loc = /* static generated by the Rust compiler */;
    mutex_init_lockdep(foo, "(rust)", (struct lock_class_key *)loc)

And when "(rust)" is encountered as lock class name, instead of printing the
string as is, lockdep will call

    lockdep_print_rust_name(loc)

which will be doing essentially

    pr_cont("foo.c:123");

Best,
Gary