ftrace: Synchronize the initialization of ftrace_opsgrafted

There's some internal state that ftrace_ops needs to have set, but since
it can be declared outside of the ftrace.c code, it calls
ftrace_ops_init() on the ops in every global function. The issue is that
if two tasks call it on the same ops at the same time it is possible to
have the initialization of one corrupt the initialization of the other
call.

Create a ops_mutex to use to synchronize every initialization of the
ftrace_ops. The mutex is taken within checking the ftrace_ops flag that
states it was initializied but the flag is checked again after the mutex
has been taken. Checking first outside the mutex allows it to shortcut
having to take the mutex. But then the check needs to be done again after
the mute is taken in case of races.

Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260902095501.6b59af20@gandalf.local.home
Fixes: f04f24fb7e48d ("ftrace, kprobes: Fix a deadlock on ftrace_regex_lock")
Reported-by: sashiko-bot@kernel.org
Close: https://lore.kernel.org/all/20260829025528.49A831F000E9@smtp.kernel.org/
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

1 files changed, 120 insertions, 0 deletions

diff --git a/rust/syn/spanned.rs b/rust/syn/spanned.rs
new file mode 100644
index 000000000..fcc93699f
--- /dev/null
+++ b/rust/syn/spanned.rs
@@ -0,0 +1,120 @@
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
+//! A trait that can provide the `Span` of the complete contents of a syntax
+//! tree node.
+//!
+//! <br>
+//!
+//! # Example
+//!
+//! Suppose in a procedural macro we have a [`Type`] that we want to assert
+//! implements the [`Sync`] trait. Maybe this is the type of one of the fields
+//! of a struct for which we are deriving a trait implementation, and we need to
+//! be able to pass a reference to one of those fields across threads.
+//!
+//! [`Type`]: crate::Type
+//! [`Sync`]: std::marker::Sync
+//!
+//! If the field type does *not* implement `Sync` as required, we want the
+//! compiler to report an error pointing out exactly which type it was.
+//!
+//! The following macro code takes a variable `ty` of type `Type` and produces a
+//! static assertion that `Sync` is implemented for that type.
+//!
+//! ```
+//! # extern crate proc_macro;
+//! #
+//! use proc_macro::TokenStream;
+//! use proc_macro2::Span;
+//! use quote::quote_spanned;
+//! use syn::Type;
+//! use syn::spanned::Spanned;
+//!
+//! # const IGNORE_TOKENS: &str = stringify! {
+//! #[proc_macro_derive(MyMacro)]
+//! # };
+//! pub fn my_macro(input: TokenStream) -> TokenStream {
+//!     # let ty = get_a_type();
+//!     /* ... */
+//!
+//!     let assert_sync = quote_spanned! {ty.span()=>
+//!         struct _AssertSync where #ty: Sync;
+//!     };
+//!
+//!     /* ... */
+//!     # input
+//! }
+//! #
+//! # fn get_a_type() -> Type {
+//! #     unimplemented!()
+//! # }
+//! ```
+//!
+//! By inserting this `assert_sync` fragment into the output code generated by
+//! our macro, the user's code will fail to compile if `ty` does not implement
+//! `Sync`. The errors they would see look like the following.
+//!
+//! ```text
+//! error[E0277]: the trait bound `*const i32: std::marker::Sync` is not satisfied
+//!   --> src/main.rs:10:21
+//!    |
+//! 10 |     bad_field: *const i32,
+//!    |                ^^^^^^^^^^ `*const i32` cannot be shared between threads safely
+//! ```
+//!
+//! In this technique, using the `Type`'s span for the error message makes the
+//! error appear in the correct place underlining the right type.
+//!
+//! <br>
+//!
+//! # Limitations
+//!
+//! The underlying [`proc_macro::Span::join`] method is nightly-only. When
+//! called from within a procedural macro in a nightly compiler, `Spanned` will
+//! use `join` to produce the intended span. When not using a nightly compiler,
+//! only the span of the *first token* of the syntax tree node is returned.
+//!
+//! In the common case of wanting to use the joined span as the span of a
+//! `syn::Error`, consider instead using [`syn::Error::new_spanned`] which is
+//! able to span the error correctly under the complete syntax tree node without
+//! needing the unstable `join`.
+//!
+//! [`syn::Error::new_spanned`]: crate::Error::new_spanned
+
+use proc_macro2::Span;
+use quote::spanned::Spanned as ToTokens;
+
+/// A trait that can provide the `Span` of the complete contents of a syntax
+/// tree node.
+///
+/// This trait is automatically implemented for all types that implement
+/// [`ToTokens`] from the `quote` crate, as well as for `Span` itself.
+///
+/// [`ToTokens`]: quote::ToTokens
+///
+/// See the [module documentation] for an example.
+///
+/// [module documentation]: self
+pub trait Spanned: private::Sealed {
+    /// Returns a `Span` covering the complete contents of this syntax tree
+    /// node, or [`Span::call_site()`] if this node is empty.
+    ///
+    /// [`Span::call_site()`]: proc_macro2::Span::call_site
+    fn span(&self) -> Span;
+}
+
+impl<T: ?Sized + ToTokens> Spanned for T {
+    fn span(&self) -> Span {
+        self.__span()
+    }
+}
+
+mod private {
+    use crate::spanned::ToTokens;
+
+    pub trait Sealed {}
+    impl<T: ?Sized + ToTokens> Sealed for T {}
+
+    #[cfg(any(feature = "full", feature = "derive"))]
+    impl Sealed for crate::QSelf {}
+}