Add multi-thread support

CMakeLists.txt

@@ -1,12 +1,23 @@
 cmake_minimum_required(VERSION 3.11)
 
+set(REFCOUNT_USE_THREADS
+    ON
+    CACHE BOOL "Weather or not RefCount should be thread aware.")
+
+if(REFCOUNT_USE_THREADS)
+  set(CMAKE_C_STANDARD 11)
+else()
   set(CMAKE_C_STANDARD 99)
+endif()
 
 project(
   refcount
   VERSION 1.0
   LANGUAGES C)
 
+# For configuring config.h (I like this name better)
+set(REFCOUNT_HAS_THREADS "${REFCOUNT_USE_THREADS}")
+
 include(FetchContent)
 FetchContent_Declare(
   ht
@@ -19,13 +30,17 @@ if(CMAKE_PROJECT_NAME STREQUAL PROJECT_NAME)
   include(CTest)
 endif()
 
-add_compile_options(-fsanitize=address,leak,undefined)
+# add_compile_options(-fsanitize=address,leak,undefined)
-add_link_options(-fsanitize=address,leak,undefined)
+# add_link_options(-fsanitize=address,leak,undefined)
+
+configure_file(include/refcount/config.h.in include/refcount/config.h @ONLY)
 
 add_library(refcount src/allocator.c src/list.c src/refcount.c)
 target_link_libraries(refcount PUBLIC ht)
 target_include_directories(refcount PUBLIC include/)
 target_compile_options(refcount PRIVATE -Wall -Wpedantic)
+target_include_directories(refcount
+                           PUBLIC "${CMAKE_CURRENT_BINARY_DIR}/include")
 
 if(CMAKE_PROJECT_NAME STREQUAL PROJECT_NAME AND BUILD_TESTING)
   add_subdirectory(test/)

README.md

@@ -1,13 +1,30 @@
 # RefCount
 
 RefCount is a reference counting (thus the name) and garbage collection library
-written in standard C99.
+written in C.
+
+RefCount has support for
+ - breaking dependency chains via garbage collection
+ - Multiple distinct contexts allowing the tracking of many types of objects,
+   each with a possibly different memory allocation implementation
+ - Safely running in multi-threaded environments
 
 ### Building
 
+The only external requirement is [ht](https://git.zander.im/Zander671/ht). If
+you build using the instructions below, CMake will automatically download and
+build the dependency for you.
+
+For multi threading support, you need a C11 compiler that supports atomic types
+and has the `<threads.h>` header. Without threading support, you only need a C99
+compiler.
+
 You can build using:
 ```sh
-cmake -B build
+cmake -B build .
+# replace with above with
+# cmake -B build -DREFCOUNT_USE_THREADS=OFF
+# to build without threads (change the "OFF" to "ON" to re-enable threads)
 make -C build
 ```
 

include/refcount/allocator.h

@@ -5,6 +5,7 @@
 #define INCLUDED_REFCOUNT_ALLOCATOR_H
 
 #include <ht.h>
+#include <refcount/config.h>
 #include <stdbool.h>
 #include <stddef.h>
 

include/refcount/config.h.in

@@ -0,0 +1,12 @@
+#ifndef INCLUDED_REFCOUNT_CONFIG_H
+#define INCLUDED_REFCOUNT_CONFIG_H
+
+#cmakedefine REFCOUNT_HAS_THREADS
+
+#if defined(REFCOUNT_HAS_THREADS) \
+    && (!defined(__STDC_VERSION__) || __STDC_VERSION__ < 201112L)
+#    error \
+        "RefCount needs to be compiled with at least C11 for thread support to work."
+#endif
+
+#endif

include/refcount/list.h

@@ -5,6 +5,7 @@
 #define INCLUDED_REFCOUNT_LIST_H
 
 #include <refcount/allocator.h>
+#include <refcount/config.h>
 #include <stdarg.h>
 #include <stddef.h> // for NULL
 

include/refcount/refcount.h

@@ -5,10 +5,16 @@
 #define INCLUDED_REFCOUNT_REFCOUNT_H
 
 #include <ht.h>
+#include <refcount/config.h>
 #include <refcount/list.h>
 #include <stdbool.h>
 #include <stdint.h>
 
+#ifdef REFCOUNT_HAS_THREADS
+#    include <stdatomic.h>
+#    include <threads.h>
+#endif
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -57,29 +63,12 @@ typedef struct RefcountContext {
     RefcountList *static_objects; //!< List of static objects registered with
                                   //!< this context.
     RefcountList *gc_roots; //!< List of garbage collector roots.
+#ifdef REFCOUNT_HAS_THREADS
+    mtx_t so_mtx; //<! Mutex protecting static_objects.
+    mtx_t gr_mtx; //<! Mutex protecting gc_roots.
+#endif
 } RefcountContext;
 
-/**
- * Enumeration of operations that may cause the debug breakpoint to be called.
- */
-typedef enum {
-    REFCOUNT_OPER_DEINIT_STATIC,
-    REFCOUNT_OPER_FLOAT,
-} RefcountOperation;
-
-/**
- * Type for a function to be called when some kind of error occurs. It should
- * take a #RefcountOperation, a #RefcountContext pointer, and the object on
- * which the error occurred.
- * @param oper The operation that failed
- * @param ctx The context the operation was performed on
- * @param obj The object that caused the failure
- */
-typedef void (*refcount_debug_callback_t)(RefcountOperation oper,
-                                          RefcountContext *ctx, void *obj);
-
-extern refcount_debug_callback_t refcount_debug_breakpoint;
-
 extern RefcountContext *refcount_default_context;
 
 RefcountContext *
@@ -90,6 +79,20 @@ refcount_make_context(size_t entry_offset,
 
 void refcount_context_destroy(RefcountContext *ctx);
 
+/**
+ * Structure holding the required information for a weak reference. That is, a
+ * reference that does not prevent an object from being de-allocated.
+ */
+typedef struct RefcountWeakref {
+#ifdef REFCOUNT_HAS_THREADS
+    _Atomic uint64_t ref_count; //!< Reference count of this weakref (atomic).
+    mtx_t mtx; //!< Mutex protecting both the weakref and it's object.
+#else
+    uint64_t ref_count; //!< Reference count of this weakref.
+#endif
+    void *data; //<! The object this weak ref references (not the entry).
+} RefcountWeakref;
+
 /**
  * Opaque struct holding reference data for an object. This should be included
  * as a non-pointer member of the struct you are trying to track, e.g.
@@ -103,8 +106,7 @@ void refcount_context_destroy(RefcountContext *ctx);
  */
 typedef struct RefcountEntry {
     bool is_static; //!< Whether the object is static.
-    RefcountList *weak_refs; //<! List of #RefcountWeakref structures for
+    RefcountWeakref *weak_ref; //<! Weakref for this entry.
-                             //<! holding thread safe weak references.
     union {
         struct {
             uint64_t ref_count; //!< The object's reference count.
@@ -115,16 +117,6 @@ typedef struct RefcountEntry {
     } impl;
 } RefcountEntry;
 
-/**
- * Structure holding the required information for a weak reference. That is, a
- * reference that does not prevent an object from being de-allocated.
- */
-typedef struct RefcountWeakref {
-    RefcountList *entry; //!< Link in the #RefcountEntry list of the object this
-                         //!< references that points to this reference.
-    void *data; //<! The object this weak ref references (not the entry).
-} RefcountWeakref;
-
 /**
  * Return a pointer to the entry associated with an object.
  * @param ctx The #RefcountContext
@@ -154,12 +146,12 @@ static inline bool refcount_context_is_static(const RefcountContext *ctx,
  * @param obj The object
  * @return The number of references the object has
  */
-static inline size_t refcount_context_num_refs(const RefcountContext *ctx,
+static inline uint64_t refcount_context_num_refs(const RefcountContext *ctx,
                                                  void *obj) {
     return REFCOUNT_OBJECT_ENTRY(ctx, obj)->impl.counted.ref_count;
 }
 
-void refcount_context_init_obj(const RefcountContext *ctx, void *obj);
+bool refcount_context_init_obj(const RefcountContext *ctx, void *obj);
 
 bool refcount_context_init_static(RefcountContext *ctx, void *obj);
 
@@ -199,19 +191,11 @@ RefcountWeakref *refcount_context_make_weakref(const RefcountContext *ctx,
 void refcount_context_destroy_weakref(const RefcountContext *ctx,
                                       RefcountWeakref *wr);
 
-/**
+bool refcount_context_weakref_is_valid(const RefcountContext *ctx,
- * Return weather the object referenced by a weak reference still exists.
+                                       RefcountWeakref *wr);
- * @param ctx The #RefcountContext
- * @param wr The weak reference
- * @return Weather the reference is still valid
- */
-static inline bool refcount_context_weakref_is_valid(const RefcountContext *ctx,
-                                                     RefcountWeakref *wr) {
-    return wr->data;
-}
 
 void *refcount_context_ref_weakref(const RefcountContext *ctx,
-                                   const RefcountWeakref *wr);
+                                   RefcountWeakref *wr);
 
 /**
  * Convenience function that calls #refcount_context_ref_weakref followed by
@@ -238,9 +222,6 @@ static inline void *refcount_context_strengthen(const RefcountContext *ctx,
 static inline RefcountWeakref *refcount_context_weaken(RefcountContext *ctx,
                                                        void *obj) {
     RefcountWeakref *wr = refcount_context_make_weakref(ctx, obj);
-    if (!wr) {
-        return NULL;
-    }
     refcount_context_unref(ctx, obj);
     return wr;
 }

src/refcount.c

@@ -10,22 +10,31 @@
 #include <stdlib.h>
 
 /**
- * Callback to be executed when a program error occurred. A sane to default is
+ * Internal magic macro.
- * to print a message and then abort. This is not used to report normal
- * recoverable errors, but rather to report fatal logic errors.
  */
-refcount_debug_callback_t refcount_debug_breakpoint = NULL;
+#define ENTRY (REFCOUNT_OBJECT_ENTRY(ctx, obj))
 
-/**
+// Some utility macros for threads
- * Call the breakpoint handler if one is installed.
+#ifdef REFCOUNT_HAS_THREADS
- * @param oper The failing operation
+#    define lock_mutex(m)   (mtx_lock(m) == thrd_success)
- * @param ctx The #RefcountContext
+#    define unlock_mutex(m) (mtx_unlock(m))
- * @param obj The object on which the operation tried to act
+#    define store_wr_count(obj, c) \
- */
+        atomic_store_explicit(obj, (c), memory_order_relaxed)
-#define BREAKPOINT(oper, ctx, obj)                                     \
+#    define inc_wr_count(obj) \
-    if (refcount_debug_breakpoint) {                                   \
+        (atomic_fetch_add_explicit(obj, 1, memory_order_relaxed))
-        refcount_debug_breakpoint(REFCOUNT_OPER_##oper, (ctx), (obj)); \
+#    define dec_wr_count(obj) \
-    }
+        (atomic_fetch_sub_explicit(obj, 1, memory_order_relaxed))
+#    define lock_entry_mtx(obj)   (lock_mutex(&obj->weak_ref->mtx))
+#    define unlock_entry_mtx(obj) (unlock_mutex(&obj->weak_ref->mtx))
+#else
+#    define lock_mutex(m)          true
+#    define unlock_mutex(m)        true
+#    define store_wr_count(obj, c) (*(obj) = (c))
+#    define inc_wr_count(obj)      ((*(obj))++)
+#    define dec_wr_count(obj)      ((*(obj))--)
+#    define lock_entry_mtx(obj)    true
+#    define unlock_entry_mtx(obj)  true
+#endif
 
 /**
  * Default context to use for functions that do not take a context. You must
@@ -56,6 +65,17 @@ refcount_make_context(size_t entry_offset,
     if (!ctx) {
         return NULL;
     }
+#ifdef REFCOUNT_HAS_THREADS
+    if (mtx_init(&ctx->so_mtx, mtx_plain) != thrd_success) {
+        refcount_free(alloc, ctx);
+        return NULL;
+    }
+    if (mtx_init(&ctx->gr_mtx, mtx_recursive) != thrd_success) {
+        refcount_free(alloc, ctx);
+        mtx_destroy(&ctx->so_mtx);
+        return NULL;
+    }
+#endif
     ctx->entry_offset = entry_offset;
     ctx->held_refs_callback = held_refs_callback;
     ctx->destroy_callback = destroy_callback;
@@ -79,28 +99,55 @@ void refcount_context_destroy(RefcountContext *ctx) {
         ctx->static_objects, refcount_context_deinit_static_as_callback, ctx,
         &ctx->alloc);
 
+#ifdef REFCOUNT_HAS_THREADS
+    mtx_destroy(&ctx->so_mtx);
+    mtx_destroy(&ctx->gr_mtx);
+#endif
+
     refcount_context_garbage_collect(ctx);
 
     refcount_free(&ctx->alloc, ctx);
 }
 
 /**
- * Internal magic macro.
+ * Initialize the weak_ref field on an object.
+ * @param ctx The #RefcountContext
+ * @param obj The object
+ * @return True on success
  */
-#define ENTRY (REFCOUNT_OBJECT_ENTRY(ctx, obj))
+static bool init_obj_weakref(const RefcountContext *ctx, void *obj) {
+    ENTRY->weak_ref = refcount_malloc(&ctx->alloc, sizeof(RefcountWeakref));
+    if (!ENTRY->weak_ref) {
+        return false;
+    }
+    ENTRY->weak_ref->data = obj;
+#ifdef REFCOUNT_HAS_THREADS
+    if (mtx_init(&ENTRY->weak_ref->mtx, mtx_recursive) != thrd_success) {
+        refcount_free(&ctx->alloc, ENTRY->weak_ref);
+        return false;
+    }
+#endif
+    store_wr_count(&ENTRY->weak_ref->ref_count, 1);
+    return true;
+}
 
 /**
  * Initialize the #RefcountEntry member of an object.
  * @param ctx The #RefcountContext
  * @param obj The object to initialize
+ * @return True on success, false on failure. Note that you don't have to do
+ * anything special to clean up the #RefcountEntry structure on failure and it
+ * is safe to call this a second time on the same object (though it will
+ * probably fail for the same reason).
  */
-void refcount_context_init_obj(const RefcountContext *ctx, void *obj) {
+bool refcount_context_init_obj(const RefcountContext *ctx, void *obj) {
     if (obj) {
         ENTRY->is_static = false;
         ENTRY->impl.counted.gc_root = NULL;
         ENTRY->impl.counted.ref_count = 0;
-        ENTRY->weak_refs = NULL;
+        init_obj_weakref(ctx, obj);
     }
+    return true;
 }
 
 /**
@@ -110,15 +157,25 @@ void refcount_context_init_obj(const RefcountContext *ctx, void *obj) {
  * @return True on success, false otherwise
  */
 bool refcount_context_init_static(RefcountContext *ctx, void *obj) {
-    ENTRY->is_static = true;
+    if (!lock_mutex(&ctx->so_mtx)) {
-    ctx->static_objects =
-        refcount_list_push_full(ctx->static_objects, obj, &ctx->alloc);
-    if (!ctx->static_objects) {
         return false;
     }
+    bool success = false;
+    ENTRY->is_static = true;
+    if (!init_obj_weakref(ctx, obj)) {
+        goto end;
+    }
+    RefcountList *new_static_objects =
+        refcount_list_push_full(ctx->static_objects, obj, &ctx->alloc);
+    if (!new_static_objects) {
+        goto end;
+    }
+    ctx->static_objects = new_static_objects;
     ENTRY->impl.static_entry = ctx->static_objects;
-    ENTRY->weak_refs = NULL;
+    success = true;
-    return true;
+end:
+    unlock_mutex(&ctx->so_mtx);
+    return success;
 }
 
 /**
@@ -137,24 +194,18 @@ static inline bool obj_held_refs(RefcountContext *ctx, void *obj,
 }
 
 /**
- * Call back used from #clear_weak_references.
+ * Remove a reference from a weakref, possibly freeing it if it's reference
- * @param wr_raw The #RefcountWeakref object
+ * count falls to zero.
- */
-static void clear_weak_reference_callback(void *wr_raw) {
-    RefcountWeakref *wr = wr_raw;
-    wr->entry = NULL;
-    wr->data = NULL;
-}
-
-/**
- * Clear all weak references of an object without freeing them.
  * @param ctx The #RefcountContext
- * @param obj The object to clear
+ * @param wr The weak reference
  */
-static inline void clear_weak_references(const RefcountContext *ctx,
+static void unref_weakref(const RefcountContext *ctx, RefcountWeakref *wr) {
-                                         void *obj) {
+    if (dec_wr_count(&wr->ref_count) == 1) {
-    refcount_list_free_full(ENTRY->weak_refs, clear_weak_reference_callback,
+#ifdef REFCOUNT_HAS_THREADS
-                            &ctx->alloc);
+        mtx_destroy(&wr->mtx);
+#endif
+        refcount_free(&ctx->alloc, wr);
+    }
 }
 
 /**
@@ -164,21 +215,32 @@ static inline void clear_weak_references(const RefcountContext *ctx,
  * @return True on success, false otherwise
  */
 bool refcount_context_deinit_static(RefcountContext *ctx, void *obj) {
-    if (refcount_context_is_static(ctx, obj)) {
+    if (!lock_mutex(&ctx->so_mtx)) {
-        RefcountList *held_refs = NULL;
-        if (!obj_held_refs(ctx, obj, &held_refs)) {
         return false;
     }
-        clear_weak_references(ctx, obj);
+    bool success = false;
+    if (!refcount_context_is_static(ctx, obj)) {
+        goto end;
+    }
+    RefcountList *held_refs = NULL;
+    if (!obj_held_refs(ctx, obj, &held_refs)) {
+        goto end;
+    }
+    if (!lock_entry_mtx(ENTRY)) {
+        refcount_list_free_full(held_refs, NULL, &ctx->alloc);
+        goto end;
+    }
+    ENTRY->weak_ref->data = NULL;
+    unref_weakref(ctx, ENTRY->weak_ref);
+    unlock_entry_mtx(ENTRY);
     ctx->static_objects = refcount_list_remove_full(
         ctx->static_objects, ENTRY->impl.static_entry, NULL, &ctx->alloc);
     refcount_list_free_with_data_full(
         held_refs, refcount_context_unref_as_callback, ctx, &ctx->alloc);
-        return true;
+    success = true;
-    } else {
+end:
-        BREAKPOINT(DEINIT_STATIC, ctx, obj)
+    unlock_mutex(&ctx->so_mtx);
-        return false;
+    return success;
-    }
 }
 
 /**
@@ -190,9 +252,14 @@ bool refcount_context_deinit_static(RefcountContext *ctx, void *obj) {
 void *refcount_context_ref(const RefcountContext *ctx, void *obj) {
     if (!obj) {
         return NULL;
-    } else if (!ENTRY->is_static) {
+    }
+    if (!lock_entry_mtx(ENTRY)) {
+        return obj;
+    }
+    if (!ENTRY->is_static) {
         ++ENTRY->impl.counted.ref_count;
     }
+    unlock_entry_mtx(ENTRY);
     return obj;
 }
 
@@ -204,15 +271,22 @@ void *refcount_context_ref(const RefcountContext *ctx, void *obj) {
  * @return True on success
  */
 static bool track_gc_root(RefcountContext *ctx, void *obj) {
+    if (!lock_mutex(&ctx->gr_mtx)) {
+        return false;
+    }
+    bool success = false;
     if (!ENTRY->impl.counted.gc_root) {
         ctx->gc_roots =
             refcount_list_push_full(ctx->gc_roots, obj, &ctx->alloc);
         if (!ctx->gc_roots) {
-            return false;
+            goto end;
         }
         ENTRY->impl.counted.gc_root = ctx->gc_roots;
     }
-    return true;
+    success = true;
+end:
+    unlock_mutex(&ctx->gr_mtx);
+    return success;
 }
 
 /**
@@ -222,9 +296,12 @@ static bool track_gc_root(RefcountContext *ctx, void *obj) {
  * @param obj The object to untrack
  */
 static void remove_gc_root(RefcountContext *ctx, void *obj) {
+    if (lock_mutex(&ctx->gr_mtx)) {
         ctx->gc_roots = refcount_list_remove_full(
             ctx->gc_roots, ENTRY->impl.counted.gc_root, NULL, &ctx->alloc);
         ENTRY->impl.counted.gc_root = NULL;
+        unlock_mutex(&ctx->gr_mtx);
+    }
 }
 
 /**
@@ -246,14 +323,23 @@ static void *unref_to_queue(RefcountContext *ctx, void *obj,
         return obj;
     } else if (obj == toplevel) {
         return NULL;
-    } else if (ENTRY->impl.counted.ref_count <= 1) {
-        *queue = refcount_list_push_full(*queue, obj, &ctx->alloc);
-        return NULL;
     }
+    if (!lock_entry_mtx(ENTRY)) {
+        // if this fails, we prefer a memory leak to causing undefined behavior
+        // and possibly crashing
+        return obj;
+    }
+    if (ENTRY->impl.counted.ref_count <= 1) {
+        *queue = refcount_list_push_full(*queue, obj, &ctx->alloc);
+        unlock_entry_mtx(ENTRY);
+        return NULL;
+    } else {
         --ENTRY->impl.counted.ref_count;
         track_gc_root(ctx, obj);
+        unlock_entry_mtx(ENTRY);
         return obj;
     }
+}
 
 /**
  * A pair of a context and double pointer to a queue. This is for internal use.
@@ -282,8 +368,13 @@ static void unref_to_queue_as_callback(void *obj, void *ctx_and_queue_raw) {
  * @param obj The object to destroy
  */
 static inline void destroy_object(RefcountContext *ctx, void *obj) {
-    clear_weak_references(ctx, obj);
+    if (!lock_entry_mtx(ENTRY)) {
+        return;
+    }
     remove_gc_root(ctx, obj);
+    ENTRY->weak_ref->data = NULL;
+    unlock_entry_mtx(ENTRY);
+    unref_weakref(ctx, ENTRY->weak_ref);
     if (ctx->destroy_callback) {
         ctx->destroy_callback(obj, ctx->user_data);
     }
@@ -351,15 +442,16 @@ void *refcount_context_float(RefcountContext *ctx, void *obj) {
         return NULL;
     } else if (ENTRY->is_static) {
         return obj;
-    } else if (!ENTRY->impl.counted.ref_count) {
+    }
-        BREAKPOINT(FLOAT, ctx, obj);
+    if (!lock_entry_mtx(ENTRY)) {
-        return obj;
+        return NULL;
     }
     if (--ENTRY->impl.counted.ref_count) {
         track_gc_root(ctx, obj);
     } else {
         remove_gc_root(ctx, obj);
     }
+    unlock_entry_mtx(ENTRY);
     return obj;
 }
 
@@ -475,6 +567,9 @@ ptrdiff_t refcount_context_garbage_collect(RefcountContext *ctx) {
         // no loops possible
         return 0;
     }
+    if (!lock_mutex(&ctx->gr_mtx)) {
+        return -1;
+    }
     ptrdiff_t total_cleared = 0;
     RefcountList *root = ctx->gc_roots;
     while (root) {
@@ -484,6 +579,7 @@ ptrdiff_t refcount_context_garbage_collect(RefcountContext *ctx) {
         }
         total_cleared += res;
     }
+    unlock_mutex(&ctx->gr_mtx);
     return total_cleared;
 }
 
@@ -495,25 +591,12 @@ ptrdiff_t refcount_context_garbage_collect(RefcountContext *ctx) {
  * NULL to indicate that the referenced object is no longer in existence.
  * @param ctx The #RefcountContext
  * @param obj The object for which to create a weak reference
- * @return The newly created weak reference, or NULL if an allocated error
+ * @return The newly created weak reference
- * occurred.
  */
 RefcountWeakref *refcount_context_make_weakref(const RefcountContext *ctx,
                                                void *obj) {
-    RefcountWeakref *wr = refcount_malloc(&ctx->alloc, sizeof(RefcountWeakref));
+    inc_wr_count(&ENTRY->weak_ref->ref_count);
-    if (!wr) {
+    return ENTRY->weak_ref;
-        return NULL;
-    }
-    RefcountList *new_weak_refs =
-        refcount_list_push_full(ENTRY->weak_refs, wr, &ctx->alloc);
-    if (!new_weak_refs) {
-        refcount_free(&ctx->alloc, wr);
-        return NULL;
-    }
-    ENTRY->weak_refs = new_weak_refs;
-    wr->entry = new_weak_refs;
-    wr->data = obj;
-    return wr;
 }
 
 /**
@@ -524,12 +607,24 @@ RefcountWeakref *refcount_context_make_weakref(const RefcountContext *ctx,
  */
 void refcount_context_destroy_weakref(const RefcountContext *ctx,
                                       RefcountWeakref *wr) {
-    if (wr->data) {
+    unref_weakref(ctx, wr);
-        void *obj = wr->data;
-        ENTRY->weak_refs = refcount_list_remove_full(
-            ENTRY->weak_refs, wr->entry, NULL, &ctx->alloc);
 }
-    refcount_free(&ctx->alloc, wr);
+
+/**
+ * Return weather the object referenced by a weak reference still exists.
+ * @param ctx The #RefcountContext
+ * @param wr The weak reference
+ * @return Weather the reference is still valid
+ */
+bool refcount_context_weakref_is_valid(const RefcountContext *ctx,
+                                       RefcountWeakref *wr) {
+    // we need the locks because accessing the data member is not atomic
+    if (!lock_mutex(&wr->mtx)) {
+        return NULL; // we can't be sure, so play it safe
+    }
+    bool is_valid = wr->data;
+    unlock_mutex(&wr->mtx);
+    return is_valid;
 }
 
 /**
@@ -540,9 +635,14 @@ void refcount_context_destroy_weakref(const RefcountContext *ctx,
  * @return The newly referenced object, or NULL
  */
 void *refcount_context_ref_weakref(const RefcountContext *ctx,
-                                   const RefcountWeakref *wr) {
+                                   RefcountWeakref *wr) {
-    if (!wr->data) {
+    if (!lock_mutex(&wr->mtx)) {
-        return NULL;
+        return NULL; // we can't be sure, so play it safe
     }
-    return refcount_context_ref(ctx, wr->data);
+    void *obj = NULL;
+    if (wr->data) {
+        obj = refcount_context_ref(ctx, wr->data);
+    }
+    unlock_mutex(&wr->mtx);
+    return obj;
 }

test/test_refcount.c

@@ -1,3 +1,4 @@
+#undef NDEBUG
 #include "alloc.h"
 
 #include <assert.h>
@@ -130,7 +131,6 @@ int main(int argc, const char **argv) {
     RefcountWeakref *x = refcount_context_weaken(c, a);
     w = refcount_context_make_weakref(c, a);
     assert(refcount_context_num_refs(c, a) == 1);
-    assert(w != x);
     refcount_context_destroy_weakref(c, x);
 
     refcount_context_ref(c, a);
@@ -138,7 +138,6 @@ int main(int argc, const char **argv) {
 
     x = refcount_context_weaken(c, a);
     assert(refcount_context_num_refs(c, a) == 1);
-    assert(w != x);
     assert(refcount_context_weakref_is_valid(c, w));
     assert(refcount_context_weakref_is_valid(c, x));