Merge #33

33: Prepare for release 0.3.0 r=bertptrs a=bertptrs

Double check

- [x] documentation
- [x] changelog
- [x] tests


Co-authored-by: Bert Peters <bert@bertptrs.nl>

.github/workflows/ci.yml

@@ -15,54 +15,34 @@ jobs:
     strategy:
       matrix:
         rust:
-          - "1.63" # minimum stable rust version
+          - "1.70" # minimum stable rust version
           - stable
           - beta
           - nightly
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
 
-      - uses: actions-rs/toolchain@v1
+      - uses: dtolnay/rust-toolchain@v1
         with:
-          profile: minimal
           toolchain: ${{ matrix.rust }}
-          override: true
           components: rustfmt, clippy
 
-      - uses: actions-rs/cargo@v1
-        with:
-          command: build
-          # --all-targets ensures that we also build the benchmarks and tests already.
-          args: --all-features --all-targets
-
-      - uses: actions-rs/cargo@v1
-        with:
-          command: test
-          args: --all-features
-
-      - uses: actions-rs/cargo@v1
-        with:
-          command: fmt
-          args: --all -- --check
-
-      - uses: actions-rs/cargo@v1
-        with:
-          command: clippy
-          args: --all-features --all-targets -- -D warnings
+      - run: cargo build --all-features --all-targets
+      - run: cargo test --all-features
+      - run: cargo fmt --all -- --check
+      - run: cargo clippy --all-features --all-targets -- -D warnings
 
   docs:
     name: Documentation build
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
 
-      - uses: actions-rs/toolchain@v1
+      - uses: dtolnay/rust-toolchain@v1
         with:
-          profile: minimal
           toolchain: nightly
-          override: true
 
       - name: Build documentation
         env:

CHANGELOG.md

@@ -6,15 +6,26 @@ adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
 
+## [0.3.0] - 2023-09-09
+
 ### Added
 
-- The minimum supported Rust version is now defined as 1.63. Previously it was undefined.
+- The minimum supported Rust version is now defined as 1.70. Previously it was undefined.
 - Wrappers for `std::sync` primitives can now be `const` constructed.
+- Add support for `std::sync::OnceLock`
+- Added backtraces of mutex allocations to the cycle report. Capturing backtraces does incur some
+  overhead, this can be mitigated by disabling the `backtraces` feature which is enabled by default.
 
 ### Breaking
 
 - Update [`parking_lot`][parking_lot] dependency to `0.12`.
-- Restructured the crate to reduce typename verbosity. For details, see: #25.
+- Restructured the crate to reduce typename verbosity. Wrapper names now match the name of the
+  primitive they wrap. Specific always/debug tracing versions have now moved to separate modules.
+  For example, `tracing_mutex::stdsync::TracingMutex` is now
+  `tracing_mutex::stdsync::tracing::Mutex`, and `tracing_mutex::stdsync::DebugMutex` is now called
+  `tracing_mutex::stdsync::Mutex`. This hopefully reduces the visual noise while reading code that
+  uses this in practice. Unwrapped primitives are reexported under `tracing_mutex::stdsync::raw` for
+  convenience.
 
 ### Fixed
 
@@ -82,7 +93,8 @@ adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 Initial release.
 
-[Unreleased]: https://github.com/bertptrs/tracing-mutex/compare/v0.2.1...HEAD
+[Unreleased]: https://github.com/bertptrs/tracing-mutex/compare/v0.3.0...HEAD
+[0.3.0]: https://github.com/bertptrs/tracing-mutex/compare/v0.2.1...v0.3.0
 [0.2.1]: https://github.com/bertptrs/tracing-mutex/compare/v0.2.0...v0.2.1
 [0.2.0]: https://github.com/bertptrs/tracing-mutex/compare/v0.1.2...v0.2.0
 [0.1.2]: https://github.com/bertptrs/tracing-mutex/compare/v0.1.1...v0.1.2

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "tracing-mutex"
-version = "0.2.1"
+version = "0.3.0"
 authors = ["Bert Peters <bert@bertptrs.nl>"]
 edition = "2021"
 license = "MIT OR Apache-2.0"
@@ -10,7 +10,7 @@ keywords = ["mutex", "rwlock", "once", "thread"]
 description = "Ensure deadlock-free mutexes by allocating in order, or else."
 readme = "README.md"
 repository = "https://github.com/bertptrs/tracing-mutex"
-rust-version = "1.63"
+rust-version = "1.70"
 
 [package.metadata.docs.rs]
 # Build docs for all features so the documentation is more complete
@@ -19,12 +19,11 @@ all-features = true
 rustdoc-args = ["--cfg", "docsrs"]
 
 [dependencies]
-lazy_static = "1"
 lock_api = { version = "0.4", optional = true }
 parking_lot = { version = "0.12", optional = true }
 
 [dev-dependencies]
-criterion = "0.3"
+criterion = "0.5"
 rand = "0.8"
 
 [[bench]]
@@ -32,6 +31,8 @@ name = "mutex"
 harness = false
 
 [features]
+default = ["backtraces"]
+backtraces = []
 # Feature names do not match crate names pending namespaced features.
 lockapi = ["lock_api"]
 parkinglot = ["parking_lot", "lockapi"]

README.md

@@ -61,7 +61,7 @@ performance penalty in your production environment, this library also offers deb
 when debug assertions are enabled, and to `Mutex` when they are not. Similar helper types are
 available for other synchronization primitives.
 
-The minimum supported Rust version is 1.63. Increasing this is not considered a breaking change, but
+The minimum supported Rust version is 1.70. Increasing this is not considered a breaking change, but
 will be avoided within semver-compatible releases if possible.
 
 ### Features

bors.toml

@@ -1,5 +1,5 @@
 status = [
-    'Rust project (1.63)',
+    'Rust project (1.70)',
     'Rust project (stable)',
     'Rust project (beta)',
     'Documentation build',

examples/mutex_cycle.rs

@@ -0,0 +1,26 @@
+//! Show what a crash looks like
+//!
+//! This shows what a traceback of a cycle detection looks like. It is expected to crash.
+use tracing_mutex::stdsync::Mutex;
+
+fn main() {
+    let a = Mutex::new(());
+    let b = Mutex::new(());
+    let c = Mutex::new(());
+
+    // Create an edge from a to b
+    {
+        let _a = a.lock();
+        let _b = b.lock();
+    }
+
+    // Create an edge from b to c
+    {
+        let _b = b.lock();
+        let _c = c.lock();
+    }
+
+    // Now crash by trying to add an edge from c to a
+    let _c = c.lock();
+    let _a = a.lock(); // This line will crash
+}

src/graph.rs

@@ -1,4 +1,5 @@
 use std::cell::Cell;
+use std::collections::hash_map::Entry;
 use std::collections::HashMap;
 use std::collections::HashSet;
 use std::hash::Hash;
@@ -19,23 +20,24 @@ type Order = usize;
 /// visibly changed.
 ///
 /// [paper]: https://whileydave.com/publications/pk07_jea/
-#[derive(Default, Debug)]
-pub struct DiGraph<V>
+#[derive(Debug)]
+pub struct DiGraph<V, E>
 where
     V: Eq + Hash + Copy,
 {
-    nodes: HashMap<V, Node<V>>,
-    /// Next topological sort order
-    next_ord: Order,
+    nodes: HashMap<V, Node<V, E>>,
+    // Instead of reordering the orders in the graph whenever a node is deleted, we maintain a list
+    // of unused ids that can be handed out later again.
+    unused_order: Vec<Order>,
 }
 
 #[derive(Debug)]
-struct Node<V>
+struct Node<V, E>
 where
     V: Eq + Hash + Clone,
 {
     in_edges: HashSet<V>,
-    out_edges: HashSet<V>,
+    out_edges: HashMap<V, E>,
     // The "Ord" field is a Cell to ensure we can update it in an immutable context.
     // `std::collections::HashMap` doesn't let you have multiple mutable references to elements, but
     // this way we can use immutable references and still update `ord`. This saves quite a few
@@ -43,7 +45,7 @@ where
     ord: Cell<Order>,
 }
 
-impl<V> DiGraph<V>
+impl<V, E> DiGraph<V, E>
 where
     V: Eq + Hash + Copy,
 {
@@ -54,12 +56,18 @@ where
     /// the node in the topological order.
     ///
     /// New nodes are appended to the end of the topological order when added.
-    fn add_node(&mut self, n: V) -> (&mut HashSet<V>, &mut HashSet<V>, Order) {
-        let next_ord = &mut self.next_ord;
+    fn add_node(&mut self, n: V) -> (&mut HashSet<V>, &mut HashMap<V, E>, Order) {
+        // need to compute next id before the call to entry() to avoid duplicate borrow of nodes
+        let fallback_id = self.nodes.len();
 
         let node = self.nodes.entry(n).or_insert_with(|| {
-            let order = *next_ord;
-            *next_ord = next_ord.checked_add(1).expect("Topological order overflow");
+            let order = if let Some(id) = self.unused_order.pop() {
+                // Reuse discarded ordering entry
+                id
+            } else {
+                // Allocate new order id
+                fallback_id
+            };
 
             Node {
                 ord: Cell::new(order),
@@ -77,9 +85,12 @@ where
             Some(Node {
                 out_edges,
                 in_edges,
-                ..
+                ord,
             }) => {
-                out_edges.into_iter().for_each(|m| {
+                // Return ordering to the pool of unused ones
+                self.unused_order.push(ord.get());
+
+                out_edges.into_keys().for_each(|m| {
                     self.nodes.get_mut(&m).unwrap().in_edges.remove(&n);
                 });
 
@@ -96,18 +107,29 @@ where
     ///
     /// Nodes, both from and to, are created as needed when creating new edges. If the new edge
     /// would introduce a cycle, the edge is rejected and `false` is returned.
-    pub(crate) fn add_edge(&mut self, x: V, y: V) -> bool {
+    ///
+    /// # Errors
+    ///
+    /// If the edge would introduce the cycle, the underlying graph is not modified and a list of
+    /// all the edge data in the would-be cycle is returned instead.
+    pub(crate) fn add_edge(&mut self, x: V, y: V, e: impl FnOnce() -> E) -> Result<(), Vec<E>>
+    where
+        E: Clone,
+    {
         if x == y {
             // self-edges are always considered cycles
-            return false;
+            return Err(Vec::new());
         }
 
         let (_, out_edges, ub) = self.add_node(x);
 
-        if !out_edges.insert(y) {
-            // Edge already exists, nothing to be done
-            return true;
-        }
+        match out_edges.entry(y) {
+            Entry::Occupied(_) => {
+                // Edge already exists, nothing to be done
+                return Ok(());
+            }
+            Entry::Vacant(entry) => entry.insert(e()),
+        };
 
         let (in_edges, _, lb) = self.add_node(y);
 
@@ -119,7 +141,7 @@ where
             let mut delta_f = Vec::new();
             let mut delta_b = Vec::new();
 
-            if !self.dfs_f(&self.nodes[&y], ub, &mut visited, &mut delta_f) {
+            if let Err(cycle) = self.dfs_f(&self.nodes[&y], ub, &mut visited, &mut delta_f) {
                 // This edge introduces a cycle, so we want to reject it and remove it from the
                 // graph again to keep the "does not contain cycles" invariant.
 
@@ -129,7 +151,7 @@ where
                 self.nodes.get_mut(&x).map(|node| node.out_edges.remove(&y));
 
                 // No edge was added
-                return false;
+                return Err(cycle);
             }
 
             // No need to check as we should've found the cycle on the forward pass
@@ -141,44 +163,49 @@ where
             self.reorder(delta_f, delta_b);
         }
 
-        true
+        Ok(())
     }
 
     /// Forwards depth-first-search
     fn dfs_f<'a>(
         &'a self,
-        n: &'a Node<V>,
+        n: &'a Node<V, E>,
         ub: Order,
         visited: &mut HashSet<V>,
-        delta_f: &mut Vec<&'a Node<V>>,
-    ) -> bool {
+        delta_f: &mut Vec<&'a Node<V, E>>,
+    ) -> Result<(), Vec<E>>
+    where
+        E: Clone,
+    {
         delta_f.push(n);
 
-        n.out_edges.iter().all(|w| {
+        for (w, e) in &n.out_edges {
             let node = &self.nodes[w];
             let ord = node.ord.get();
 
             if ord == ub {
                 // Found a cycle
-                false
+                return Err(vec![e.clone()]);
             } else if !visited.contains(w) && ord < ub {
                 // Need to check recursively
                 visited.insert(*w);
-                self.dfs_f(node, ub, visited, delta_f)
-            } else {
-                // Already seen this one or not interesting
-                true
+                if let Err(mut chain) = self.dfs_f(node, ub, visited, delta_f) {
+                    chain.push(e.clone());
+                    return Err(chain);
+                }
             }
-        })
+        }
+
+        Ok(())
     }
 
     /// Backwards depth-first-search
     fn dfs_b<'a>(
         &'a self,
-        n: &'a Node<V>,
+        n: &'a Node<V, E>,
         lb: Order,
         visited: &mut HashSet<V>,
-        delta_b: &mut Vec<&'a Node<V>>,
+        delta_b: &mut Vec<&'a Node<V, E>>,
     ) {
         delta_b.push(n);
 
@@ -192,7 +219,7 @@ where
         }
     }
 
-    fn reorder(&self, mut delta_f: Vec<&Node<V>>, mut delta_b: Vec<&Node<V>>) {
+    fn reorder(&self, mut delta_f: Vec<&Node<V, E>>, mut delta_b: Vec<&Node<V, E>>) {
         self.sort(&mut delta_f);
         self.sort(&mut delta_b);
 
@@ -213,12 +240,25 @@ where
         }
     }
 
-    fn sort(&self, ids: &mut [&Node<V>]) {
+    fn sort(&self, ids: &mut [&Node<V, E>]) {
         // Can use unstable sort because mutex ids should not be equal
         ids.sort_unstable_by_key(|v| &v.ord);
     }
 }
 
+// Manual `Default` impl as derive causes unnecessarily strong bounds.
+impl<V, E> Default for DiGraph<V, E>
+where
+    V: Eq + Hash + Copy,
+{
+    fn default() -> Self {
+        Self {
+            nodes: Default::default(),
+            unused_order: Default::default(),
+        }
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use rand::seq::SliceRandom;
@@ -226,12 +266,14 @@ mod tests {
 
     use super::*;
 
+    fn nop() {}
+
     #[test]
     fn test_no_self_cycle() {
         // Regression test for https://github.com/bertptrs/tracing-mutex/issues/7
         let mut graph = DiGraph::default();
 
-        assert!(!graph.add_edge(1, 1));
+        assert!(graph.add_edge(1, 1, nop).is_err());
     }
 
     #[test]
@@ -239,16 +281,16 @@ mod tests {
         let mut graph = DiGraph::default();
 
         // Add some safe edges
-        assert!(graph.add_edge(0, 1));
-        assert!(graph.add_edge(1, 2));
-        assert!(graph.add_edge(2, 3));
-        assert!(graph.add_edge(4, 2));
+        assert!(graph.add_edge(0, 1, nop).is_ok());
+        assert!(graph.add_edge(1, 2, nop).is_ok());
+        assert!(graph.add_edge(2, 3, nop).is_ok());
+        assert!(graph.add_edge(4, 2, nop).is_ok());
 
         // Try to add an edge that introduces a cycle
-        assert!(!graph.add_edge(3, 1));
+        assert!(graph.add_edge(3, 1, nop).is_err());
 
         // Add an edge that should reorder 0 to be after 4
-        assert!(graph.add_edge(4, 0));
+        assert!(graph.add_edge(4, 0, nop).is_ok());
     }
 
     /// Fuzz the DiGraph implementation by adding a bunch of valid edges.
@@ -256,7 +298,7 @@ mod tests {
     /// This test generates all possible forward edges in a 100-node graph consisting of natural
     /// numbers, shuffles them, then adds them to the graph. This will always be a valid directed,
     /// acyclic graph because there is a trivial order (the natural numbers) but because the edges
-    /// are added in a random order the DiGraph will still occassionally need to reorder nodes.
+    /// are added in a random order the DiGraph will still occasionally need to reorder nodes.
     #[test]
     fn fuzz_digraph() {
         // Note: this fuzzer is quadratic in the number of nodes, so this cannot be too large or it
@@ -277,7 +319,7 @@ mod tests {
         let mut graph = DiGraph::default();
 
         for (x, y) in edges {
-            assert!(graph.add_edge(x, y));
+            assert!(graph.add_edge(x, y, nop).is_ok());
         }
     }
 }

src/lib.rs

@@ -18,8 +18,23 @@
 //! # Structure
 //!
 //! Each module in this crate exposes wrappers for a specific base-mutex with dependency trakcing
-//! added. For now, that is limited to [`stdsync`] which provides wrappers for the base locks in the
-//! standard library. More back-ends may be added as features in the future.
+//! added. This includes [`stdsync`] which provides wrappers for the base locks in the standard
+//! library, and more depending on enabled compile-time features. More back-ends may be added as
+//! features in the future.
+//!
+//! # Feature flags
+//!
+//! `tracing-mutex` uses feature flags to reduce the impact of this crate on both your compile time
+//! and runtime overhead. Below are the available flags. Modules are annotated with the features
+//! they require.
+//!
+//! - `backtraces`: Enables capturing backtraces of mutex dependencies, to make it easier to
+//!   determine what sequence of events would trigger a deadlock. This is enabled by default, but if
+//!   the performance overhead is unaccceptable, it can be disabled by disabling default features.
+//!
+//! - `lockapi`: Enables the wrapper lock for [`lock_api`][lock_api] locks
+//!
+//! - `parkinglot`: Enables wrapper types for [`parking_lot`][parking_lot] mutexes
 //!
 //! # Performance considerations
 //!
@@ -44,29 +59,34 @@
 //! (such as [`stdsync::Mutex`]) which evaluate to a tracing mutex when debug assertions are
 //! enabled, and to the underlying mutex when they're not.
 //!
+//! For ease of debugging, this crate will, by default, capture a backtrace when establishing a new
+//! dependency between two mutexes. This has an additional overhead of over 60%. If this additional
+//! debugging aid is not required, it can be disabled by disabling default features.
+//!
 //! [paper]: https://whileydave.com/publications/pk07_jea/
+//! [lock_api]: https://docs.rs/lock_api/0.4/lock_api/index.html
+//! [parking_lot]: https://docs.rs/parking_lot/0.12.1/parking_lot/
 #![cfg_attr(docsrs, feature(doc_cfg))]
 use std::cell::RefCell;
-use std::cell::UnsafeCell;
 use std::fmt;
 use std::marker::PhantomData;
-use std::mem::MaybeUninit;
 use std::ops::Deref;
 use std::ops::DerefMut;
 use std::sync::atomic::AtomicUsize;
 use std::sync::atomic::Ordering;
 use std::sync::Mutex;
 use std::sync::MutexGuard;
-use std::sync::Once;
+use std::sync::OnceLock;
 use std::sync::PoisonError;
 
-use lazy_static::lazy_static;
 #[cfg(feature = "lockapi")]
 #[cfg_attr(docsrs, doc(cfg(feature = "lockapi")))]
 pub use lock_api;
 #[cfg(feature = "parkinglot")]
 #[cfg_attr(docsrs, doc(cfg(feature = "parkinglot")))]
 pub use parking_lot;
+use reporting::Dep;
+use reporting::Reportable;
 
 use crate::graph::DiGraph;
 
@@ -77,13 +97,9 @@ pub mod lockapi;
 #[cfg(feature = "parkinglot")]
 #[cfg_attr(docsrs, doc(cfg(feature = "parkinglot")))]
 pub mod parkinglot;
+mod reporting;
 pub mod stdsync;
 
-/// Counter for Mutex IDs. Atomic avoids the need for locking.
-///
-/// Should be part of the `MutexID` impl but static items are not yet a thing.
-static ID_SEQUENCE: AtomicUsize = AtomicUsize::new(0);
-
 thread_local! {
     /// Stack to track which locks are held
     ///
@@ -92,10 +108,6 @@ thread_local! {
     static HELD_LOCKS: RefCell<Vec<usize>> = RefCell::new(Vec::new());
 }
 
-lazy_static! {
-    static ref DEPENDENCY_GRAPH: Mutex<DiGraph<usize>> = Default::default();
-}
-
 /// Dedicated ID type for Mutexes
 ///
 /// # Unstable
@@ -114,6 +126,9 @@ impl MutexId {
     /// This function may panic when there are no more mutex IDs available. The number of mutex ids
     /// is `usize::MAX - 1` which should be plenty for most practical applications.
     pub fn new() -> Self {
+        // Counter for Mutex IDs. Atomic avoids the need for locking.
+        static ID_SEQUENCE: AtomicUsize = AtomicUsize::new(0);
+
         ID_SEQUENCE
             .fetch_update(Ordering::SeqCst, Ordering::SeqCst, |id| id.checked_add(1))
             .map(Self)
@@ -146,19 +161,18 @@ impl MutexId {
     ///
     /// This method panics if the new dependency would introduce a cycle.
     pub fn mark_held(&self) {
-        let creates_cycle = HELD_LOCKS.with(|locks| {
+        let opt_cycle = HELD_LOCKS.with(|locks| {
             if let Some(&previous) = locks.borrow().last() {
                 let mut graph = get_dependency_graph();
 
-                !graph.add_edge(previous, self.value())
+                graph.add_edge(previous, self.value(), Dep::capture).err()
             } else {
-                false
+                None
             }
         });
 
-        if creates_cycle {
-            // Panic without holding the lock to avoid needlessly poisoning it
-            panic!("Mutex order graph should not have cycles");
+        if let Some(cycle) = opt_cycle {
+            panic!("{}", Dep::panic_message(&cycle))
         }
 
         HELD_LOCKS.with(|locks| locks.borrow_mut().push(self.value()));
@@ -207,17 +221,13 @@ impl Drop for MutexId {
 ///
 /// This type can be largely replaced once std::lazy gets stabilized.
 struct LazyMutexId {
-    inner: UnsafeCell<MaybeUninit<MutexId>>,
-    setter: Once,
-    _marker: PhantomData<MutexId>,
+    inner: OnceLock<MutexId>,
 }
 
 impl LazyMutexId {
     pub const fn new() -> Self {
         Self {
-            inner: UnsafeCell::new(MaybeUninit::uninit()),
-            setter: Once::new(),
-            _marker: PhantomData,
+            inner: OnceLock::new(),
         }
     }
 }
@@ -234,44 +244,11 @@ impl Default for LazyMutexId {
     }
 }
 
-/// Safety: the UnsafeCell is guaranteed to only be accessed mutably from a `Once`.
-unsafe impl Sync for LazyMutexId {}
-
 impl Deref for LazyMutexId {
     type Target = MutexId;
 
     fn deref(&self) -> &Self::Target {
-        self.setter.call_once(|| {
-            // Safety: this function is only called once, so only one mutable reference should exist
-            // at a time.
-            unsafe {
-                *self.inner.get() = MaybeUninit::new(MutexId::new());
-            }
-        });
-
-        // Safety: after the above Once runs, there are no longer any mutable references, so we can
-        // hand this out safely.
-        //
-        // Explanation of this monstrosity:
-        //
-        // - Get a pointer to the data from the UnsafeCell
-        // - Dereference that to get a reference to the underlying MaybeUninit
-        // - Use as_ptr on MaybeUninit to get a pointer to the initialized MutexID
-        // - Dereference the pointer to turn in into a reference as intended.
-        //
-        // This should get slightly nicer once `maybe_uninit_extra` is stabilized.
-        unsafe { &*((*self.inner.get()).as_ptr()) }
-    }
-}
-
-impl Drop for LazyMutexId {
-    fn drop(&mut self) {
-        if self.setter.is_completed() {
-            // We have a valid mutex ID and need to drop it
-
-            // Safety: we know that this pointer is valid because the initializer has successfully run.
-            unsafe { (*self.inner.get()).assume_init_drop() };
-        }
+        self.inner.get_or_init(MutexId::new)
     }
 }
 
@@ -306,8 +283,11 @@ impl<'a> Drop for BorrowedMutex<'a> {
 }
 
 /// Get a reference to the current dependency graph
-fn get_dependency_graph() -> impl DerefMut<Target = DiGraph<usize>> {
+fn get_dependency_graph() -> impl DerefMut<Target = DiGraph<usize, Dep>> {
+    static DEPENDENCY_GRAPH: OnceLock<Mutex<DiGraph<usize, Dep>>> = OnceLock::new();
+
     DEPENDENCY_GRAPH
+        .get_or_init(Default::default)
         .lock()
         .unwrap_or_else(PoisonError::into_inner)
 }
@@ -335,11 +315,11 @@ mod tests {
         let c = LazyMutexId::new();
 
         let mut graph = get_dependency_graph();
-        assert!(graph.add_edge(a.value(), b.value()));
-        assert!(graph.add_edge(b.value(), c.value()));
+        assert!(graph.add_edge(a.value(), b.value(), Dep::capture).is_ok());
+        assert!(graph.add_edge(b.value(), c.value(), Dep::capture).is_ok());
 
         // Creating an edge c → a should fail as it introduces a cycle.
-        assert!(!graph.add_edge(c.value(), a.value()));
+        assert!(graph.add_edge(c.value(), a.value(), Dep::capture).is_err());
 
         // Drop graph handle so we can drop vertices without deadlocking
         drop(graph);
@@ -347,7 +327,9 @@ mod tests {
         drop(b);
 
         // If b's destructor correctly ran correctly we can now add an edge from c to a.
-        assert!(get_dependency_graph().add_edge(c.value(), a.value()));
+        assert!(get_dependency_graph()
+            .add_edge(c.value(), a.value(), Dep::capture)
+            .is_ok());
     }
 
     /// Test creating a cycle, then panicking.

src/reporting.rs

@@ -0,0 +1,64 @@
+//! Cycle reporting primitives
+//!
+//! This module exposes [`Dep`], which resolves to either something that tracks dependencies or to
+//! something that doesn't.  It should only be assumed to implement the [`Reportable`] trait.
+use std::backtrace::Backtrace;
+use std::borrow::Cow;
+use std::fmt::Write;
+use std::sync::Arc;
+
+#[cfg(feature = "backtraces")]
+pub type Dep = MutexDep<Arc<Backtrace>>;
+#[cfg(not(feature = "backtraces"))]
+pub type Dep = MutexDep<()>;
+
+// Base message to be reported when cycle is detected
+const BASE_MESSAGE: &str = "Found cycle in mutex dependency graph:";
+
+pub trait Reportable: Clone {
+    /// Capture the current state
+    fn capture() -> Self;
+
+    /// Format a trace of state for human readable consumption.
+    fn panic_message(trace: &[Self]) -> Cow<'static, str>;
+}
+
+#[derive(Clone)]
+pub struct MutexDep<T>(T);
+
+/// Use a unit as tracing data: no tracing.
+///
+/// This should have no runtime overhead for capturing traces and should therefore be cheap enough
+/// for most purposes.
+impl Reportable for MutexDep<()> {
+    fn capture() -> Self {
+        Self(())
+    }
+
+    fn panic_message(_trace: &[Self]) -> Cow<'static, str> {
+        Cow::Borrowed(BASE_MESSAGE)
+    }
+}
+
+/// Use a full backtrace as tracing data
+///
+/// Capture the entire backtrace which may be expensive. This implementation does not force capture
+/// in the event that backtraces are disabled at runtime, so the exact overhead can still be
+/// controlled a little.
+///
+/// N.B. the [`Backtrace`] needs to be wrapped in an Arc as backtraces are not [`Clone`].
+impl Reportable for MutexDep<Arc<Backtrace>> {
+    fn capture() -> Self {
+        Self(Arc::new(Backtrace::capture()))
+    }
+
+    fn panic_message(trace: &[Self]) -> Cow<'static, str> {
+        let mut message = format!("{BASE_MESSAGE}\n");
+
+        for entry in trace {
+            let _ = writeln!(message, "{}", entry.0);
+        }
+
+        message.into()
+    }
+}

src/stdsync.rs

@@ -21,10 +21,14 @@
 pub use std::sync as raw;
 
 #[cfg(not(debug_assertions))]
-pub use std::sync::{Condvar, Mutex, MutexGuard, Once, RwLock, RwLockReadGuard, RwLockWriteGuard};
+pub use std::sync::{
+    Condvar, Mutex, MutexGuard, Once, OnceLock, RwLock, RwLockReadGuard, RwLockWriteGuard,
+};
 
 #[cfg(debug_assertions)]
-pub use tracing::{Condvar, Mutex, MutexGuard, Once, RwLock, RwLockReadGuard, RwLockWriteGuard};
+pub use tracing::{
+    Condvar, Mutex, MutexGuard, Once, OnceLock, RwLock, RwLockReadGuard, RwLockWriteGuard,
+};
 
 /// Dependency tracing versions of [`std::sync`].
 pub mod tracing {
@@ -427,8 +431,8 @@ pub mod tracing {
 
     /// Wrapper around [`std::sync::Once`].
     ///
-    /// Refer to the [crate-level][`crate`] documentaiton for the differences between this struct and
-    /// the one it wraps.
+    /// Refer to the [crate-level][`crate`] documentaiton for the differences between this struct
+    /// and the one it wraps.
     #[derive(Debug)]
     pub struct Once {
         inner: sync::Once,
@@ -479,6 +483,142 @@ pub mod tracing {
         }
     }
 
+    /// Wrapper for [`std::sync::OnceLock`]
+    ///
+    /// The exact locking behaviour of [`std::sync::OnceLock`] is currently undefined, but may
+    /// deadlock in the event of reentrant initialization attempts. This wrapper participates in
+    /// cycle detection as normal and will therefore panic in the event of reentrancy.
+    ///
+    /// Most of this primitive's methods do not involve locking and as such are simply passed
+    /// through to the inner implementation.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tracing_mutex::stdsync::tracing::OnceLock;
+    ///
+    /// static LOCK: OnceLock<i32> = OnceLock::new();
+    /// assert!(LOCK.get().is_none());
+    ///
+    /// std::thread::spawn(|| {
+    ///    let value: &i32 = LOCK.get_or_init(|| 42);
+    ///    assert_eq!(value, &42);
+    /// }).join().unwrap();
+    ///
+    /// let value: Option<&i32> = LOCK.get();
+    /// assert_eq!(value, Some(&42));
+    /// ```
+    #[derive(Debug)]
+    pub struct OnceLock<T> {
+        id: LazyMutexId,
+        inner: sync::OnceLock<T>,
+    }
+
+    // N.B. this impl inlines everything that directly calls the inner implementation as there
+    // should be 0 overhead to doing so.
+    impl<T> OnceLock<T> {
+        /// Creates a new empty cell
+        pub const fn new() -> Self {
+            Self {
+                id: LazyMutexId::new(),
+                inner: sync::OnceLock::new(),
+            }
+        }
+
+        /// Gets a reference to the underlying value.
+        ///
+        /// This method does not attempt to lock and therefore does not participate in cycle
+        /// detection.
+        #[inline]
+        pub fn get(&self) -> Option<&T> {
+            self.inner.get()
+        }
+
+        /// Gets a mutable reference to the underlying value.
+        ///
+        /// This method does not attempt to lock and therefore does not participate in cycle
+        /// detection.
+        #[inline]
+        pub fn get_mut(&mut self) -> Option<&mut T> {
+            self.inner.get_mut()
+        }
+
+        /// Sets the contents of this cell to the underlying value
+        ///
+        /// As this method may block until initialization is complete, it participates in cycle
+        /// detection.
+        pub fn set(&self, value: T) -> Result<(), T> {
+            let _guard = self.id.get_borrowed();
+
+            self.inner.set(value)
+        }
+
+        /// Gets the contents of the cell, initializing it with `f` if the cell was empty.
+        ///
+        /// This method participates in cycle detection. Reentrancy is considered a cycle.
+        pub fn get_or_init<F>(&self, f: F) -> &T
+        where
+            F: FnOnce() -> T,
+        {
+            let _guard = self.id.get_borrowed();
+            self.inner.get_or_init(f)
+        }
+
+        /// Takes the value out of this `OnceLock`, moving it back to an uninitialized state.
+        ///
+        /// This method does not attempt to lock and therefore does not participate in cycle
+        /// detection.
+        #[inline]
+        pub fn take(&mut self) -> Option<T> {
+            self.inner.take()
+        }
+
+        /// Consumes the `OnceLock`, returning the wrapped value. Returns None if the cell was
+        /// empty.
+        ///
+        /// This method does not attempt to lock and therefore does not participate in cycle
+        /// detection.
+        #[inline]
+        pub fn into_inner(mut self) -> Option<T> {
+            self.take()
+        }
+    }
+
+    impl<T> Default for OnceLock<T> {
+        #[inline]
+        fn default() -> Self {
+            Self::new()
+        }
+    }
+
+    impl<T: PartialEq> PartialEq for OnceLock<T> {
+        #[inline]
+        fn eq(&self, other: &Self) -> bool {
+            self.inner == other.inner
+        }
+    }
+
+    impl<T: Eq> Eq for OnceLock<T> {}
+
+    impl<T: Clone> Clone for OnceLock<T> {
+        fn clone(&self) -> Self {
+            Self {
+                id: LazyMutexId::new(),
+                inner: self.inner.clone(),
+            }
+        }
+    }
+
+    impl<T> From<T> for OnceLock<T> {
+        #[inline]
+        fn from(value: T) -> Self {
+            Self {
+                id: LazyMutexId::new(),
+                inner: sync::OnceLock::from(value),
+            }
+        }
+    }
+
     #[cfg(test)]
     mod tests {
         use std::sync::Arc;
@@ -556,7 +696,7 @@ pub mod tracing {
         }
 
         #[test]
-        #[should_panic(expected = "Mutex order graph should not have cycles")]
+        #[should_panic(expected = "Found cycle in mutex dependency graph")]
         fn test_detect_cycle() {
             let a = Mutex::new(());
             let b = Mutex::new(());