Merge patch series "Add support for print exactly once"

Tomonori writes:

  "This adds the Rust equivalent of the C's DO_ONCE_LITE and pr_*_once
   macros.

   A proposal for this feature was made in the past [1], but it didn't
   reach consensus on the implementation and wasn't merged.

   Unlike the previous proposal, this implements the C's DO_ONCE_LITE
   mechanism using a single atomic variable. While it would be possible
   to implement the feature entirely as a Rust macro, the functionality
   that can be implemented as regular functions has been extracted and
   implemented as the OnceLite struct for better code readability.

   To make it clear that this feature is intended solely for
   print-related functionality (just like in C), the implementation is
   placed in print.rs. If an equivalent of std::sync::Once is needed in
   the future, it should be based on SetOnce rather than OnceLite. Unlike
   std::sync::Once and SetOnce, OnceLite only manages two states:
   incomplete and complete.

   The do_once_lite macro places the OnceLite object in the .data..once
   section, which may be zeroed by memset at any time. While this means
   tear reads might happen, OnceLite only manages two states (zero and
   non-zero), so no actual problem occurs in practice.

   OnceLite currently uses Atomic<i32>, but may be changed to use
   Atomic<i8> [2] when it becomes available."

Link: https://lore.kernel.org/rust-for-linux/20241126-pr_once_macros-v4-0-410b8ca9643e@tuta.io/ [1]
Link: https://lore.kernel.org/rust-for-linux/20251115050305.3872412-1-fujita.tomonori@gmail.com/ [2]
Link: https://patch.msgid.link/20251117002452.4068692-1-fujita.tomonori@gmail.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

rust/kernel/print.rs

@@ -11,6 +11,11 @@ use crate::{
     fmt,
     prelude::*,
     str::RawFormatter,
+    sync::atomic::{
+        Atomic,
+        AtomicType,
+        Relaxed, //
+    },
 };
 
 // Called from `vsprintf` with format specifier `%pA`.
@@ -423,3 +428,151 @@ macro_rules! pr_cont (
         $crate::print_macro!($crate::print::format_strings::CONT, true, $($arg)*)
     )
 );
+
+/// A lightweight `call_once` primitive.
+///
+/// This structure provides the Rust equivalent of the kernel's `DO_ONCE_LITE` macro.
+/// While it would be possible to implement the feature entirely as a Rust macro,
+/// the functionality that can be implemented as regular functions has been
+/// extracted and implemented as the `OnceLite` struct for better code maintainability.
+pub struct OnceLite(Atomic<State>);
+
+#[derive(Clone, Copy, PartialEq, Eq)]
+#[repr(i32)]
+enum State {
+    Incomplete = 0,
+    Complete = 1,
+}
+
+// SAFETY: `State` and `i32` has the same size and alignment, and it's round-trip
+// transmutable to `i32`.
+unsafe impl AtomicType for State {
+    type Repr = i32;
+}
+
+impl OnceLite {
+    /// Creates a new [`OnceLite`] in the incomplete state.
+    #[inline(always)]
+    #[allow(clippy::new_without_default)]
+    pub const fn new() -> Self {
+        OnceLite(Atomic::new(State::Incomplete))
+    }
+
+    /// Calls the provided function exactly once.
+    ///
+    /// There is no other synchronization between two `call_once()`s
+    /// except that only one will execute `f`, in other words, callers
+    /// should not use a failed `call_once()` as a proof that another
+    /// `call_once()` has already finished and the effect is observable
+    /// to this thread.
+    pub fn call_once<F>(&self, f: F) -> bool
+    where
+        F: FnOnce(),
+    {
+        // Avoid expensive cmpxchg if already completed.
+        // ORDERING: `Relaxed` is used here since no synchronization is required.
+        let old = self.0.load(Relaxed);
+        if old == State::Complete {
+            return false;
+        }
+
+        // ORDERING: `Relaxed` is used here since no synchronization is required.
+        let old = self.0.xchg(State::Complete, Relaxed);
+        if old == State::Complete {
+            return false;
+        }
+
+        f();
+        true
+    }
+}
+
+/// Run the given function exactly once.
+///
+/// This is equivalent to the kernel's `DO_ONCE_LITE` macro.
+///
+/// # Examples
+///
+/// ```
+/// kernel::do_once_lite! {
+///     kernel::pr_info!("This will be printed only once\n");
+/// };
+/// ```
+#[macro_export]
+macro_rules! do_once_lite {
+    { $($e:tt)* } => {{
+        #[link_section = ".data..once"]
+        static ONCE: $crate::print::OnceLite = $crate::print::OnceLite::new();
+        ONCE.call_once(|| { $($e)* });
+    }};
+}
+
+/// Prints an emergency-level message (level 0) only once.
+///
+/// Equivalent to the kernel's `pr_emerg_once` macro.
+#[macro_export]
+macro_rules! pr_emerg_once (
+    ($($arg:tt)*) => (
+        $crate::do_once_lite! { $crate::pr_emerg!($($arg)*) }
+    )
+);
+
+/// Prints an alert-level message (level 1) only once.
+///
+/// Equivalent to the kernel's `pr_alert_once` macro.
+#[macro_export]
+macro_rules! pr_alert_once (
+    ($($arg:tt)*) => (
+        $crate::do_once_lite! { $crate::pr_alert!($($arg)*) }
+    )
+);
+
+/// Prints a critical-level message (level 2) only once.
+///
+/// Equivalent to the kernel's `pr_crit_once` macro.
+#[macro_export]
+macro_rules! pr_crit_once (
+    ($($arg:tt)*) => (
+        $crate::do_once_lite! { $crate::pr_crit!($($arg)*) }
+    )
+);
+
+/// Prints an error-level message (level 3) only once.
+///
+/// Equivalent to the kernel's `pr_err_once` macro.
+#[macro_export]
+macro_rules! pr_err_once (
+    ($($arg:tt)*) => (
+        $crate::do_once_lite! { $crate::pr_err!($($arg)*) }
+    )
+);
+
+/// Prints a warning-level message (level 4) only once.
+///
+/// Equivalent to the kernel's `pr_warn_once` macro.
+#[macro_export]
+macro_rules! pr_warn_once (
+    ($($arg:tt)*) => (
+        $crate::do_once_lite! { $crate::pr_warn!($($arg)*) }
+    )
+);
+
+/// Prints a notice-level message (level 5) only once.
+///
+/// Equivalent to the kernel's `pr_notice_once` macro.
+#[macro_export]
+macro_rules! pr_notice_once (
+    ($($arg:tt)*) => (
+        $crate::do_once_lite! { $crate::pr_notice!($($arg)*) }
+    )
+);
+
+/// Prints an info-level message (level 6) only once.
+///
+/// Equivalent to the kernel's `pr_info_once` macro.
+#[macro_export]
+macro_rules! pr_info_once (
+    ($($arg:tt)*) => (
+        $crate::do_once_lite! { $crate::pr_info!($($arg)*) }
+    )
+);