diff --git a/runtime/caml/gc_ctrl.h b/runtime/caml/gc_ctrl.h
index c56470b4f9..223bd50549 100644
--- a/runtime/caml/gc_ctrl.h
+++ b/runtime/caml/gc_ctrl.h
@@ -20,7 +20,7 @@
 
 #include "misc.h"
 
-CAMLextern uintnat caml_max_stack_wsize;
+CAMLextern atomic_uintnat caml_max_stack_wsize;
 CAMLextern uintnat caml_fiber_wsz;
 CAMLextern uintnat caml_major_cycles_completed;
 
diff --git a/runtime/fiber.c b/runtime/fiber.c
index 083d1ef4fb..5a5ed2e995 100644
--- a/runtime/fiber.c
+++ b/runtime/fiber.c
@@ -496,8 +496,9 @@ int caml_try_realloc_stack(asize_t required_space)
   old_stack = Caml_state->current_stack;
   stack_used = Stack_high(old_stack) - (value*)old_stack->sp;
   wsize = Stack_high(old_stack) - Stack_base(old_stack);
+  uintnat max_stack_wsize = caml_max_stack_wsize;
   do {
-    if (wsize >= caml_max_stack_wsize) return 0;
+    if (wsize >= max_stack_wsize) return 0;
     wsize *= 2;
   } while (wsize < stack_used + required_space);
 
diff --git a/runtime/gc_ctrl.c b/runtime/gc_ctrl.c
index 436789591e..6e399af350 100644
--- a/runtime/gc_ctrl.c
+++ b/runtime/gc_ctrl.c
@@ -38,7 +38,7 @@
 #include "caml/startup.h"
 #include "caml/fail.h"
 
-uintnat caml_max_stack_wsize;
+atomic_uintnat caml_max_stack_wsize;
 uintnat caml_fiber_wsz;
 
 extern uintnat caml_percent_free; /* see major_gc.c */
@@ -335,7 +335,7 @@ void caml_init_gc (void)
   caml_percent_free = norm_pfree (caml_params->init_percent_free);
   caml_gc_log ("Initial stack limit: %"
                ARCH_INTNAT_PRINTF_FORMAT "uk bytes",
-               caml_max_stack_wsize / 1024 * sizeof (value));
+               caml_params->init_max_stack_wsz / 1024 * sizeof (value));
 
   caml_custom_major_ratio =
       norm_custom_maj (caml_params->init_custom_major_ratio);