Fix data race in GC_heapsize_at_forced_unmap variable

(a cherry-pick of commit 856760844 from 'release-8_2')

Issue #488 (bdwgc).

Write to GC_heapsize_at_forced_unmap should be only performed when
holding the allocation lock.

* alloc.c (GC_heapsize_at_forced_unmap): Move the definition upper to
be before GC_try_to_collect_general(); add comment.
* alloc.c (GC_try_to_collect_general): Set GC_heapsize_at_forced_unmap
value (after LOCK) if force_unmap; move comment from
GC_heapsize_at_forced_unmap().
* alloc.c (GC_try_to_collect_general): Remove GC_ATTR_UNUSED for
force_unmap.
* alloc.c (GC_gcollect_and_unmap): Do not set
GC_heapsize_at_forced_unmap.

1 files changed, 9 insertions, 6 deletions

diff --git a/alloc.c b/alloc.c
index 12498bbb..9e7f9d24 100644
--- a/alloc.c
+++ b/alloc.c
@@ -1134,9 +1134,12 @@ STATIC void GC_finish_collection(void)
 #   endif
 }
 
+STATIC word GC_heapsize_at_forced_unmap = 0;
+                                /* accessed with the allocation lock held */
+
 /* If stop_func == 0 then GC_default_stop_func is used instead.         */
 STATIC GC_bool GC_try_to_collect_general(GC_stop_func stop_func,
-                                         GC_bool force_unmap GC_ATTR_UNUSED)
+                                         GC_bool force_unmap)
 {
     GC_bool result;
     IF_USE_MUNMAP(int old_unmap_threshold;)
@@ -1147,6 +1150,11 @@ STATIC GC_bool GC_try_to_collect_general(GC_stop_func stop_func,
     if (GC_debugging_started) GC_print_all_smashed();
     GC_INVOKE_FINALIZERS();
     LOCK();
+    if (force_unmap) {
+      /* Record current heap size to make heap growth more conservative */
+      /* afterwards (as if the heap is growing from zero size again).   */
+      GC_heapsize_at_forced_unmap = GC_heapsize;
+    }
     DISABLE_CANCEL(cancel_state);
 #   ifdef USE_MUNMAP
       old_unmap_threshold = GC_unmap_threshold;
@@ -1185,13 +1193,8 @@ GC_API void GC_CALL GC_gcollect(void)
     if (GC_have_errors) GC_print_all_errors();
 }
 
-STATIC word GC_heapsize_at_forced_unmap = 0;
-
 GC_API void GC_CALL GC_gcollect_and_unmap(void)
 {
-    /* Record current heap size to make heap growth more conservative   */
-    /* afterwards (as if the heap is growing from zero size again).     */
-    GC_heapsize_at_forced_unmap = GC_heapsize;
     /* Collect and force memory unmapping to OS. */
     (void)GC_try_to_collect_general(GC_never_stop_func, TRUE);
 }