Merge pull request ocaml#12969 from fabbing/race_gc_tracing_free_stack

Fix a data race in `caml_darken_cont`
gasche · Feb 12, 2024 · ef62c57 · ef62c57
2 parents ec52575 + 7435700
commit ef62c57
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diff --git a/Changes b/Changes
@@ -87,6 +87,9 @@ _______________
   command-line of the toplevel.
   (Nicolás Ojeda Bär, review by Florian Angeletti, report by Daniel Bünzli)
 
+- #12969: Fix a data race in caml_darken_cont
+  (Fabrice Buoro and Olivier Nicole, review by Gabriel Scherer and Miod Vallat)
+
 OCaml 5.2.0
 ------------
 

diff --git a/runtime/major_gc.c b/runtime/major_gc.c
@@ -1184,8 +1184,12 @@ void caml_darken_cont(value cont)
     SPIN_WAIT {
       header_t hd = atomic_load_relaxed(Hp_atomic_val(cont));
       CAMLassert(!Has_status_hd(hd, caml_global_heap_state.GARBAGE));
-      if (Has_status_hd(hd, caml_global_heap_state.MARKED))
-        break;
+      if (Has_status_hd(hd, caml_global_heap_state.MARKED)) {
+        /* Perform an acquire load to synchronize with the marking domain */
+        hd = atomic_load_acquire(Hp_atomic_val(cont));
+        if (Has_status_hd(hd, caml_global_heap_state.MARKED))
+          break;
+      }
       if (Has_status_hd(hd, caml_global_heap_state.UNMARKED) &&
           atomic_compare_exchange_strong(
               Hp_atomic_val(cont), &hd,