diff --git a/thread.c b/thread.c
index 01007b1862..7bcf64741c 100644
--- a/thread.c
+++ b/thread.c
@@ -79,8 +79,9 @@ st_delete_wrap(st_table *table, st_data_t key)
 
 #define THREAD_SYSTEM_DEPENDENT_IMPLEMENTATION
 
-static void set_unblock_function(rb_thread_t *th, rb_unblock_function_t *func, void *ptr,
-				 rb_unblock_function_t **oldfunc, void **oldptr);
+static void set_unblock_function(rb_thread_t *th, rb_unblock_function_t *func, void *arg,
+				 struct rb_unblock_callback *old);
+static void reset_unblock_function(rb_thread_t *th, const struct rb_unblock_callback *old);
 
 #define GVL_UNLOCK_BEGIN() do { \
   rb_thread_t *_th_stored = GET_THREAD(); \
@@ -95,9 +96,8 @@ static void set_unblock_function(rb_thread_t *th, rb_unblock_function_t *func, v
 #define BLOCKING_REGION(exec, ubf, ubfarg) do { \
     rb_thread_t *__th = GET_THREAD(); \
     int __prev_status = __th->status; \
-    rb_unblock_function_t *__oldubf; \
-    void *__oldubfarg; \
-    set_unblock_function(__th, ubf, ubfarg, &__oldubf, &__oldubfarg); \
+    struct rb_unblock_callback __oldubf; \
+    set_unblock_function(__th, ubf, ubfarg, &__oldubf); \
     __th->status = THREAD_STOPPED; \
     thread_debug("enter blocking region (%p)\n", __th); \
     GVL_UNLOCK_BEGIN(); {\
@@ -106,7 +106,7 @@ static void set_unblock_function(rb_thread_t *th, rb_unblock_function_t *func, v
     GVL_UNLOCK_END(); \
     thread_debug("leave blocking region (%p)\n", __th); \
     remove_signal_thread_list(__th); \
-    set_unblock_function(__th, __oldubf, __oldubfarg, 0, 0); \
+    reset_unblock_function(__th, &__oldubf); \
     if (__th->status == THREAD_STOPPED) { \
 	__th->status = __prev_status; \
     } \
@@ -195,7 +195,7 @@ rb_thread_debug(const char *fmt, ...)
 
 static void
 set_unblock_function(rb_thread_t *th, rb_unblock_function_t *func, void *arg,
-		     rb_unblock_function_t **oldfunc, void **oldarg)
+		     struct rb_unblock_callback *old)
 {
   check_ints:
     RUBY_VM_CHECK_INTS(); /* check signal or so */
@@ -205,21 +205,28 @@ set_unblock_function(rb_thread_t *th, rb_unblock_function_t *func, void *arg,
 	goto check_ints;
     }
     else {
-	if (oldfunc) *oldfunc = th->unblock_function;
-	if (oldarg) *oldarg = th->unblock_function_arg;
-	th->unblock_function = func;
-	th->unblock_function_arg = arg;
+	if (old) *old = th->unblock;
+	th->unblock.func = func;
+	th->unblock.arg = arg;
     }
     native_mutex_unlock(&th->interrupt_lock);
 }
 
+static void
+reset_unblock_function(rb_thread_t *th, const struct rb_unblock_callback *old)
+{
+    native_mutex_lock(&th->interrupt_lock);
+    th->unblock = *old;
+    native_mutex_unlock(&th->interrupt_lock);
+}
+
 static void
 rb_thread_interrupt(rb_thread_t *th)
 {
     native_mutex_lock(&th->interrupt_lock);
     RUBY_VM_SET_INTERRUPT(th);
-    if (th->unblock_function) {
-	(th->unblock_function)(th->unblock_function_arg);
+    if (th->unblock.func) {
+	(th->unblock.func)(th->unblock.arg);
     }
     else {
 	/* none */
diff --git a/thread_pthread.c b/thread_pthread.c
index 58ce378841..e5a6566267 100644
--- a/thread_pthread.c
+++ b/thread_pthread.c
@@ -424,8 +424,8 @@ native_sleep(rb_thread_t *th, struct timeval *tv)
     GVL_UNLOCK_BEGIN();
     {
 	pthread_mutex_lock(&th->interrupt_lock);
-	th->unblock_function = ubf_pthread_cond_signal;
-	th->unblock_function_arg = th;
+	th->unblock.func = ubf_pthread_cond_signal;
+	th->unblock.arg = th;
 
 	if (RUBY_VM_INTERRUPTED(th)) {
 	    /* interrupted.  return immediate */
@@ -451,8 +451,8 @@ native_sleep(rb_thread_t *th, struct timeval *tv)
 		thread_debug("native_sleep: pthread_cond_timedwait end (%d)\n", r);
 	    }
 	}
-	th->unblock_function = 0;
-	th->unblock_function_arg = 0;
+	th->unblock.func = 0;
+	th->unblock.arg = 0;
 
 	pthread_mutex_unlock(&th->interrupt_lock);
 	th->status = prev_status;
diff --git a/thread_win32.c b/thread_win32.c
index a83ede0b94..63425511c1 100644
--- a/thread_win32.c
+++ b/thread_win32.c
@@ -220,8 +220,8 @@ native_sleep(rb_thread_t *th, struct timeval *tv)
 	int status = th->status;
 
 	th->status = THREAD_STOPPED;
-	th->unblock_function = ubf_handle;
-	th->unblock_function_arg = th;
+	th->unblock.func = ubf_handle;
+	th->unblock.arg = th;
 
 	if (RUBY_VM_INTERRUPTED(th)) {
 	    /* interrupted.  return immediate */
@@ -232,8 +232,8 @@ native_sleep(rb_thread_t *th, struct timeval *tv)
 	    thread_debug("native_sleep done (%lu)\n", ret);
 	}
 
-	th->unblock_function = 0;
-	th->unblock_function_arg = 0;
+	th->unblock.func = 0;
+	th->unblock.arg = 0;
 	th->status = status;
     }
     GVL_UNLOCK_END();
diff --git a/vm_core.h b/vm_core.h
index 8f39c68995..f0fdafef2d 100644
--- a/vm_core.h
+++ b/vm_core.h
@@ -373,6 +373,11 @@ struct rb_vm_trap_tag {
 #define RUBY_VM_VALUE_CACHE_SIZE 0x1000
 #define USE_VALUE_CACHE 0
 
+struct rb_unblock_callback {
+    rb_unblock_function_t *func;
+    void *arg;
+};
+
 typedef struct rb_thread_struct rb_thread_t;
 
 struct rb_thread_struct
@@ -418,9 +423,8 @@ struct rb_thread_struct
     int exec_signal;
 
     int interrupt_flag;
-    rb_unblock_function_t *unblock_function;
-    void *unblock_function_arg;
     rb_thread_lock_t interrupt_lock;
+    struct rb_unblock_callback unblock;
 
     struct rb_vm_tag *tag;
     struct rb_vm_trap_tag *trap_tag;