linux-kernel - Re: [PATCH RFC 07/11] rust: sync: Implement dynamic lockdep class creation

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Date:   Fri, 14 Jul 2023 16:56:17 -0300
From:   Martin Rodriguez Reboredo <yakoyoku@...il.com>
To:     Asahi Lina <lina@...hilina.net>, Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Wedson Almeida Filho <wedsonaf@...il.com>,
        Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        Benno Lossin <benno.lossin@...ton.me>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nicolas Schier <nicolas@...sle.eu>, Tom Rix <trix@...hat.com>,
        Daniel Vetter <daniel@...ll.ch>
Cc:     Hector Martin <marcan@...can.st>, Sven Peter <sven@...npeter.dev>,
        Alyssa Rosenzweig <alyssa@...enzweig.io>,
        asahi@...ts.linux.dev, rust-for-linux@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-kbuild@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: Re: [PATCH RFC 07/11] rust: sync: Implement dynamic lockdep class
 creation

On 7/14/23 06:13, Asahi Lina wrote:
> Using macros to create lock classes all over the place is unergonomic,
> and makes it impossible to add new features that require lock classes to
> code such as Arc<> without changing all callers.
> 
> Rust has the ability to track the caller's identity by file/line/column
> number, and we can use that to dynamically generate lock classes
> instead.
> 
> Signed-off-by: Asahi Lina <lina@...hilina.net>
> ---
> [...]
> +
> +const LOCK_CLASS_BUCKETS: usize = 1024;
> +
> +#[track_caller]
> +fn caller_lock_class_inner() -> Result<&'static DynLockClassKey> {
> +    // This is just a hack to make the below static array initialization work.
> +    #[allow(clippy::declare_interior_mutable_const)]
> +    const ATOMIC_PTR: AtomicPtr<Mutex<Vec<&'static DynLockClassKey>>> =
> +        AtomicPtr::new(core::ptr::null_mut());
> +
> +    #[allow(clippy::complexity)]
> +    static LOCK_CLASSES: [AtomicPtr<Mutex<Vec<&'static DynLockClassKey>>>; LOCK_CLASS_BUCKETS] =
> +        [ATOMIC_PTR; LOCK_CLASS_BUCKETS];
> +
> +    let loc = core::panic::Location::caller();
> +    let loc_key = LocationKey::new(loc);
> +
> +    let index = (loc_key.hash % (LOCK_CLASS_BUCKETS as u64)) as usize;
> +    let slot = &LOCK_CLASSES[index];
> +
> +    let mut ptr = slot.load(Ordering::Relaxed);
> +    if ptr.is_null() {
> +        let new_element = Box::pin_init(new_mutex!(Vec::new()))?;
> +
> +        if let Err(e) = slot.compare_exchange(
> +            core::ptr::null_mut(),
> +            // SAFETY: We never move out of this Box
> +            Box::into_raw(unsafe { Pin::into_inner_unchecked(new_element) }),
> +            Ordering::Relaxed,
> +            Ordering::Relaxed,
> +        ) {
> +            // SAFETY: We just got this pointer from `into_raw()`
> +            unsafe { Box::from_raw(e) };
> +        }
> +
> +        ptr = slot.load(Ordering::Relaxed);
> +        assert!(!ptr.is_null());
> +    }
> +
> +    // SAFETY: This mutex was either just created above or previously allocated,
> +    // and we never free these objects so the pointer is guaranteed to be valid.
> +    let mut guard = unsafe { (*ptr).lock() };
> +
> +    for i in guard.iter() {
> +        if i.loc == loc_key {
> +            return Ok(i);
> +        }
> +    }
> +
> +    // We immediately leak the class, so it becomes 'static
> +    let new_class = Box::leak(Box::try_new(DynLockClassKey {
> +        key: Opaque::zeroed(),
> +        loc: loc_key,
> +        name: CString::try_from_fmt(fmt!("{}:{}:{}", loc.file(), loc.line(), loc.column()))?,
> +    })?);
> +
> +    // SAFETY: This is safe to call with a pointer to a dynamically allocated lockdep key,
> +    // and we never free the objects so it is safe to never unregister the key.
> +    unsafe { bindings::lockdep_register_key(new_class.key.get()) };
> +
> +    guard.try_push(new_class)?;
> +
> +    Ok(new_class)
> +}
> +
> [...]

Is there any problem if we have many `DynLockClassKey`s leaked or not?