erts: Fix race bug between export fun creation and code loading

Symptom: SEGV crash on ARM in delete_code() -> export_list().
Could probably happen on other machines as well.

Problem: Staging export table was iterated in an unsafe way
while an entry was added for a new export fun.

Solution: Correct write order and some memory barriers.

1 files changed, 14 insertions, 0 deletions

diff --git a/erts/emulator/beam/index.h b/erts/emulator/beam/index.h
index 0a109d8699..532dec0168 100644
--- a/erts/emulator/beam/index.h
+++ b/erts/emulator/beam/index.h
@@ -65,6 +65,7 @@ void index_erase_latest_from(IndexTable*, Uint ix);
 
 ERTS_GLB_INLINE int index_put(IndexTable*, void*);
 ERTS_GLB_INLINE IndexSlot* erts_index_lookup(IndexTable*, Uint);
+ERTS_GLB_INLINE int erts_index_num_entries(IndexTable* t);
 
 #if ERTS_GLB_INLINE_INCL_FUNC_DEF
 
@@ -78,6 +79,19 @@ erts_index_lookup(IndexTable* t, Uint ix)
 {
     return t->seg_table[ix>>INDEX_PAGE_SHIFT][ix&INDEX_PAGE_MASK];
 }
+
+ERTS_GLB_INLINE int erts_index_num_entries(IndexTable* t)
+{
+    int ret = t->entries;
+    /*
+     * Do a read barrier here to allow lock free iteration
+     * on tables where entries are never erased.
+     * index_put_entry() does matching write barrier.
+     */
+    ERTS_SMP_READ_MEMORY_BARRIER;
+    return ret;
+}
+
 #endif
 
 #endif