diff --git a/libguile/atomics-internal.h b/libguile/atomics-internal.h
index e15ea3564..048484576 100644
--- a/libguile/atomics-internal.h
+++ b/libguile/atomics-internal.h
@@ -1,7 +1,7 @@
 #ifndef SCM_ATOMICS_INTERNAL_H
 #define SCM_ATOMICS_INTERNAL_H
 
-/* Copyright 2016,2018-2019
+/* Copyright 2016,2018-2019,2025
      Free Software Foundation, Inc.
 
    This file is part of Guile.
@@ -58,6 +58,12 @@ scm_atomic_ref_pointer (void **loc)
   return (void *) atomic_load (a_loc);
 }
 static inline void
+scm_atomic_set_bits (scm_t_bits *loc, scm_t_bits val)
+{
+  atomic_uintptr_t *a_loc = (atomic_uintptr_t *) loc;
+  atomic_store (a_loc, val);
+}
+static inline void
 scm_atomic_set_scm (SCM *loc, SCM val)
 {
   atomic_uintptr_t *a_loc = (atomic_uintptr_t *) loc;
@@ -136,6 +142,14 @@ scm_atomic_ref_pointer (void **loc)
   return ret;
 }
 
+static inline void
+scm_atomic_set_bits (scm_t_bits *loc, scm_t_bits val)
+{
+  scm_i_pthread_mutex_lock (&atomics_lock);
+  *loc = val;
+  scm_i_pthread_mutex_unlock (&atomics_lock);
+}
+
 static inline void
 scm_atomic_set_scm (SCM *loc, SCM val)
 {
diff --git a/libguile/smob.c b/libguile/smob.c
index 8e4da9adb..4fba079d9 100644
--- a/libguile/smob.c
+++ b/libguile/smob.c
@@ -1,4 +1,4 @@
-/* Copyright 1995-1996,1998-2001,2003-2004,2006,2009-2013,2015,2018
+/* Copyright 1995-1996,1998-2001,2003-2004,2006,2009-2013,2015,2018,2025
      Free Software Foundation, Inc.
 
    This file is part of Guile.
@@ -29,6 +29,7 @@
 #include <stdlib.h>
 
 #include "async.h"
+#include "atomics-internal.h"
 #include "bdw-gc.h"
 #include "finalizers.h"
 #include "goops.h"
@@ -375,45 +376,37 @@ scm_gc_mark (SCM o)
 }
 
 
-static void*
-clear_smobnum (void *ptr)
+/* Finalize SMOB by calling its SMOB type's free function, if any.  */
+void
+scm_i_finalize_smob (struct scm_thread *thread, SCM smob)
 {
-  SCM smob;
-  scm_t_bits smobnum;
+  scm_t_bits *first_word_loc = SCM_UNPACK_POINTER (smob);
+  scm_t_bits first_word = *first_word_loc;
+  scm_t_bits smobnum = SCM_TC2SMOBNUM (first_word & SCM_SMOB_TYPE_MASK);
 
-  smob = SCM_PACK_POINTER (ptr);
-
-  smobnum = SCM_SMOBNUM (smob);
   /* Frob the object's type in place, re-setting it to be the "finalized
      smob" type.  This will prevent other routines from accessing its
      internals in a way that assumes that the smob data is valid.  This
      is notably the case for SMOB's own "mark" procedure, if any; as the
-     finalizer runs without the alloc lock, it's possible for a GC to
+     finalizer is invoked by the mutator, it's possible for a GC to
      occur while it's running, in which case the object is alive and yet
      its data is invalid.  */
-  SCM_SET_SMOB_DATA_0 (smob, SCM_SMOB_DATA_0 (smob) & ~(scm_t_bits) 0xff00);
+  scm_t_bits finalized_word = first_word & ~(scm_t_bits) 0xff00;
+  scm_atomic_set_bits (first_word_loc, finalized_word);
 
-  return (void *) smobnum;
+#if 0
+  printf ("finalizing SMOB %p (smobnum: %u)\n", first_word_loc, smobnum);
+#endif
+
+  size_t (* free_smob) (SCM) = scm_smobs[smobnum].free;
+  if (free_smob)
+    free_smob (smob);
 }
 
-/* Finalize SMOB by calling its SMOB type's free function, if any.  */
 static void
 finalize_smob (void *ptr, void *data)
 {
-  SCM smob;
-  scm_t_bits smobnum;
-  size_t (* free_smob) (SCM);
-
-  smob = SCM_PACK_POINTER (ptr);
-  smobnum = (scm_t_bits) GC_call_with_alloc_lock (clear_smobnum, ptr);
-
-#if 0
-  printf ("finalizing SMOB %p (smobnum: %u)\n", ptr, smobnum);
-#endif
-
-  free_smob = scm_smobs[smobnum].free;
-  if (free_smob)
-    free_smob (smob);
+  return scm_i_finalize_smob (SCM_I_CURRENT_THREAD, PTR2SCM (ptr));
 }
 
 /* Return a SMOB with typecode TC.  The SMOB type corresponding to TC may
diff --git a/libguile/smob.h b/libguile/smob.h
index d137b3278..990ac057b 100644
--- a/libguile/smob.h
+++ b/libguile/smob.h
@@ -1,7 +1,7 @@
 #ifndef SCM_SMOB_H
 #define SCM_SMOB_H
 
-/* Copyright 1995-1996,1998-2001,2004,2006,2009-2012,2015,2018
+/* Copyright 1995-1996,1998-2001,2004,2006,2009-2012,2015,2018,2025
      Free Software Foundation, Inc.
 
    This file is part of Guile.
@@ -256,6 +256,8 @@ SCM_API void scm_assert_smob_type (scm_t_bits tag, SCM val);
 
 SCM_API SCM scm_make_smob (scm_t_bits tc);
 
+SCM_INTERNAL void scm_i_finalize_smob (struct scm_thread *thread, SCM obj);
+
 SCM_API void scm_smob_prehistory (void);
 
 #endif  /* SCM_SMOB_H */