[sgen] Fix call to `InterlockedCompareExchange()`.

This fixes random crashes on `sgen-weakref-stress.exe`.

diff --git a/mono/sgen/sgen-gchandles.c b/mono/sgen/sgen-gchandles.c
index 34d2c0f6ec47e996afd5cd0584e92c18d67980a0..11216ffd21d410171c5a4ae171e5eff1338c99c4 100644 (file)
--- a/mono/sgen/sgen-gchandles.c
+++ b/mono/sgen/sgen-gchandles.c
@@ -260,18 +260,28 @@ retry:
                goto retry;
        }
        handles->slot_hint = index;
-       bucketize (index, &bucket, &offset);
-       if (!try_occupy_slot (handles, bucket, offset, obj, track))
-               goto retry;
-       /* If a GC happens shortly after a new bucket is allocated, the entire
-        * bucket could be scanned even though it's mostly empty. To avoid this, we
-        * track the maximum index seen so far, so that we can skip the empty slots.
+
+       /*
+        * If a GC happens shortly after a new bucket is allocated, the entire
+        * bucket could be scanned even though it's mostly empty. To avoid this,
+        * we track the maximum index seen so far, so that we can skip the empty
+        * slots.
+        *
+        * Note that we update `max_index` before we even try occupying the
+        * slot.  If we did it the other way around and a GC happened in
+        * between, the GC wouldn't know that the slot was occupied.  This is
+        * not a huge deal since `obj` is on the stack and thus pinned anyway,
+        * but hopefully some day it won't be anymore.
         */
        do {
                max_index = handles->max_index;
                if (index <= max_index)
                        break;
-       } while (!InterlockedCompareExchange ((volatile gint32 *)&handles->max_index, index, max_index));
+       } while (InterlockedCompareExchange ((volatile gint32 *)&handles->max_index, index, max_index) != max_index);
+
+       bucketize (index, &bucket, &offset);
+       if (!try_occupy_slot (handles, bucket, offset, obj, track))
+               goto retry;
 #ifdef HEAVY_STATISTICS
        InterlockedIncrement ((volatile gint32 *)&stat_gc_handles_allocated);
        if (stat_gc_handles_allocated > stat_gc_handles_max_allocated)