diff --git a/newsfragments/2975.added.md b/newsfragments/2975.added.md
new file mode 100644
index 00000000000..7a3da624f2a
--- /dev/null
+++ b/newsfragments/2975.added.md
@@ -0,0 +1 @@
+Add `GILProtected<T>` to mediate concurrent access to a value using Python's global interpreter lock (GIL).
diff --git a/src/gil.rs b/src/gil.rs
index 678f12eecad..b898800856b 100644
--- a/src/gil.rs
+++ b/src/gil.rs
@@ -494,6 +494,29 @@ impl EnsureGIL {
     }
 }
 
+/// Value with concurrent access protected by the GIL.
+///
+/// This is similar to [`Mutex<T>`][std::sync::Mutex] but the lock used is Python's global interpreter lock (GIL).
+pub struct GILProtected<T> {
+    value: T,
+}
+
+impl<T> GILProtected<T> {
+    /// Place the given value under the protection of the GIL.
+    pub const fn new(value: T) -> Self {
+        Self { value }
+    }
+
+    /// Gain access to the inner value by giving proof of having acquired the GIL.
+    pub fn with_gil<'py>(&'py self, _py: Python<'py>) -> &'py T {
+        &self.value
+    }
+}
+
+unsafe impl<T> Send for GILProtected<T> where T: Send {}
+
+unsafe impl<T> Sync for GILProtected<T> where T: Send {}
+
 #[cfg(test)]
 mod tests {
     use super::{gil_is_acquired, GILPool, GIL_COUNT, OWNED_OBJECTS, POOL};
diff --git a/src/impl_/pyclass/lazy_type_object.rs b/src/impl_/pyclass/lazy_type_object.rs
index 8990bba3717..d7eadc02696 100644
--- a/src/impl_/pyclass/lazy_type_object.rs
+++ b/src/impl_/pyclass/lazy_type_object.rs
@@ -1,16 +1,15 @@
 use std::{
     borrow::Cow,
+    cell::RefCell,
     ffi::CStr,
     marker::PhantomData,
     thread::{self, ThreadId},
 };
 
-use parking_lot::{const_mutex, Mutex};
-
 use crate::{
-    exceptions::PyRuntimeError, ffi, once_cell::GILOnceCell, pyclass::create_type_object,
-    types::PyType, AsPyPointer, IntoPyPointer, Py, PyClass, PyErr, PyMethodDefType, PyObject,
-    PyResult, Python,
+    exceptions::PyRuntimeError, ffi, gil::GILProtected, once_cell::GILOnceCell,
+    pyclass::create_type_object, types::PyType, AsPyPointer, IntoPyPointer, Py, PyClass, PyErr,
+    PyMethodDefType, PyObject, PyResult, Python,
 };
 
 use super::PyClassItemsIter;
@@ -24,7 +23,7 @@ struct LazyTypeObjectInner {
     value: GILOnceCell<Py<PyType>>,
     // Threads which have begun initialization of the `tp_dict`. Used for
     // reentrant initialization detection.
-    initializing_threads: Mutex<Vec<ThreadId>>,
+    initializing_threads: GILProtected<RefCell<Vec<ThreadId>>>,
     tp_dict_filled: GILOnceCell<()>,
 }
 
@@ -34,7 +33,7 @@ impl<T> LazyTypeObject<T> {
         LazyTypeObject(
             LazyTypeObjectInner {
                 value: GILOnceCell::new(),
-                initializing_threads: const_mutex(Vec::new()),
+                initializing_threads: GILProtected::new(RefCell::new(Vec::new())),
                 tp_dict_filled: GILOnceCell::new(),
             },
             PhantomData,
@@ -109,7 +108,7 @@ impl LazyTypeObjectInner {
 
         let thread_id = thread::current().id();
         {
-            let mut threads = self.initializing_threads.lock();
+            let mut threads = self.initializing_threads.with_gil(py).borrow_mut();
             if threads.contains(&thread_id) {
                 // Reentrant call: just return the type object, even if the
                 // `tp_dict` is not filled yet.
@@ -119,18 +118,20 @@ impl LazyTypeObjectInner {
         }
 
         struct InitializationGuard<'a> {
-            initializing_threads: &'a Mutex<Vec<ThreadId>>,
+            initializing_threads: &'a GILProtected<RefCell<Vec<ThreadId>>>,
+            py: Python<'a>,
             thread_id: ThreadId,
         }
         impl Drop for InitializationGuard<'_> {
             fn drop(&mut self) {
-                let mut threads = self.initializing_threads.lock();
+                let mut threads = self.initializing_threads.with_gil(self.py).borrow_mut();
                 threads.retain(|id| *id != self.thread_id);
             }
         }
 
         let guard = InitializationGuard {
             initializing_threads: &self.initializing_threads,
+            py,
             thread_id,
         };
 
@@ -170,7 +171,7 @@ impl LazyTypeObjectInner {
             // Initialization successfully complete, can clear the thread list.
             // (No further calls to get_or_init() will try to init, on any thread.)
             std::mem::forget(guard);
-            *self.initializing_threads.lock() = Vec::new();
+            self.initializing_threads.with_gil(py).replace(Vec::new());
             result
         });
 
diff --git a/src/lib.rs b/src/lib.rs
index 940f061e6b3..cfa08a6df3c 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -304,7 +304,7 @@ pub use crate::conversion::{
 pub use crate::err::{PyDowncastError, PyErr, PyErrArguments, PyResult};
 #[cfg(not(PyPy))]
 pub use crate::gil::{prepare_freethreaded_python, with_embedded_python_interpreter};
-pub use crate::gil::{GILGuard, GILPool};
+pub use crate::gil::{GILGuard, GILPool, GILProtected};
 pub use crate::instance::{Py, PyNativeType, PyObject};
 pub use crate::marker::Python;
 pub use crate::pycell::{PyCell, PyRef, PyRefMut};
diff --git a/src/pyclass/create_type_object.rs b/src/pyclass/create_type_object.rs
index 2409a0f1ff2..882f3a5dbe0 100644
--- a/src/pyclass/create_type_object.rs
+++ b/src/pyclass/create_type_object.rs
@@ -335,9 +335,10 @@ impl PyTypeBuilder {
             unsafe { self.push_slot(ffi::Py_tp_new, no_constructor_defined as *mut c_void) }
         }
 
-        if !self.has_dealloc {
-            panic!("PyTypeBuilder requires you to specify slot ffi::Py_tp_dealloc");
-        }
+        assert!(
+            self.has_dealloc,
+            "PyTypeBuilder requires you to specify slot ffi::Py_tp_dealloc"
+        );
 
         if self.has_clear && !self.has_traverse {
             return Err(PyTypeError::new_err(format!(