diff --git a/racket/src/racket/gc2/newgc.c b/racket/src/racket/gc2/newgc.c
index 4ea848be4e..9b339e946c 100644
--- a/racket/src/racket/gc2/newgc.c
+++ b/racket/src/racket/gc2/newgc.c
@@ -780,6 +780,9 @@ static void NewGC_initialize(NewGC *newgc, NewGC *inheritgc, NewGC *parentgc) {
 #ifdef MZ_USE_PLACES
   mzrt_mutex_create(&newgc->child_total_lock);
 #endif
+#ifdef USE_MUTEX_FOR_MODIFIED_PAGES
+  mzrt_mutex_create(&newgc->modified_pages_lock);
+#endif
 }
 
 /* NOTE This method sets the constructed GC as the new Thread Specific GC. */
@@ -1089,11 +1092,17 @@ static int designate_modified_gc(NewGC *gc, void *p)
   }
 
   if (page) {
+#ifdef USE_MUTEX_FOR_MODIFIED_PAGES
+    mzrt_mutex_lock(gc->modified_pages_lock);
+#endif
     page->mprotected = 0;
     mmu_write_unprotect_page(gc->mmu, page->addr, real_page_size(page), page_mmu_type(page), &page->mmu_src_block);
     if (!page->back_pointers)
       set_has_back_pointers(gc, page);
     gc->modified_unprotects++;
+#ifdef USE_MUTEX_FOR_MODIFIED_PAGES
+    mzrt_mutex_unlock(gc->modified_pages_lock);
+#endif
     errno = saved_errno;
     return 1;
   } else {
diff --git a/racket/src/racket/gc2/newgc.h b/racket/src/racket/gc2/newgc.h
index 080adae925..3346006b60 100644
--- a/racket/src/racket/gc2/newgc.h
+++ b/racket/src/racket/gc2/newgc.h
@@ -53,6 +53,10 @@ enum {
   AGE_GEN_INC  = 4  /* used for naming a finalizer set */
 };
 
+#if defined(MZ_USE_FUTURES) && !defined(OS_X)
+# define USE_MUTEX_FOR_MODIFIED_PAGES
+#endif
+
 typedef struct mpage {
   struct mpage *next;
   struct mpage *prev;
@@ -396,7 +400,11 @@ typedef struct NewGC {
   uintptr_t child_gc_cumulative;
   uintptr_t child_gc_max;
 
-  uintptr_t place_memory_limit; /* set to propagate a custodian limit from a parent place */  
+  uintptr_t place_memory_limit; /* set to propagate a custodian limit from a parent place */
+
+#ifdef USE_MUTEX_FOR_MODIFIED_PAGES
+  mzrt_mutex *modified_pages_lock;
+#endif
 
 #if MZ_GC_BACKTRACE
   void *bt_source;
diff --git a/racket/src/racket/gc2/vm_osx.c b/racket/src/racket/gc2/vm_osx.c
index cad91620b6..43eb39f75a 100644
--- a/racket/src/racket/gc2/vm_osx.c
+++ b/racket/src/racket/gc2/vm_osx.c
@@ -427,6 +427,10 @@ static void macosx_init_exception_handler(int isMASTERGC)
 {
   kern_return_t retval;
 
+  /* Note: the `designate_modified` function relies on the fact that
+     all exceptions (at least within a place) go through the same
+     handler thread, so it can skip the lock on modified pages. */
+
   if (!isMASTERGC) {
     GC_attach_current_thread_exceptions_to_handler();
     return;