Pin array buffer in RARRAY_PTR_USE

This prevents the underlying buffer from being moved by the GC when the
pointer to the buffer is passed to native C code unaware of object
movement (such as `qsort_r`).

include/ruby/internal/core/rarray.h

@@ -320,13 +320,37 @@ rb_array_const_ptr(VALUE a)
  * This is an implementation detail of #RARRAY_PTR_USE. People do not use it
  * directly.
  */
+#if USE_MMTK
+// Defined in mmtk_support.c
+bool rb_mmtk_enabled_p(void);
+void rb_mmtk_pin_array_buffer(VALUE array, volatile VALUE *stack_slot);
+
+// When using MMTk, we need to pin the underlying buffer if the array is not embedded becuase the
+// buffer is in the GC heap. Otherwise, if C calls back to Ruby, and Ruby triggers GCm and GC
+// moves the array buffer, the C function will be operating on the old address of the buffer.
+#define RBIMPL_RARRAY_STMT(ary, var, expr) do { \
+ RBIMPL_ASSERT_TYPE((ary), RUBY_T_ARRAY); \
+ const VALUE rbimpl_ary = (ary); \
+ volatile VALUE rb_mmtk_impl_pinned; \
+ if (rb_mmtk_enabled_p()) { \
+ rb_mmtk_pin_array_buffer(ary, &rb_mmtk_impl_pinned); \
+ } else { \
+ rb_mmtk_impl_pinned = 0; \
+ } \
+ VALUE *var = rb_ary_ptr_use_start(rbimpl_ary); \
+ expr; \
+ rb_ary_ptr_use_end(rbimpl_ary); \
+ rb_mmtk_impl_pinned = 0; \
+} while (0)
+#else
 #define RBIMPL_RARRAY_STMT(ary, var, expr) do { \
  RBIMPL_ASSERT_TYPE((ary), RUBY_T_ARRAY); \
  const VALUE rbimpl_ary = (ary); \
  VALUE *var = rb_ary_ptr_use_start(rbimpl_ary); \
  expr; \
  rb_ary_ptr_use_end(rbimpl_ary); \
 } while (0)
+#endif
 
 /**
  * Declares a section of code where raw pointers are used. In case you need to

internal/mmtk_support.h

@@ -91,6 +91,10 @@ void rb_mmtk_scan_offsetted_strbuf_field(char** field, bool update);
 rb_mmtk_objbuf_t* rb_mmtk_new_objbuf(size_t capa);
 VALUE* rb_mmtk_objbuf_to_elems(rb_mmtk_objbuf_t* objbuf);
 
+// Object pinning
+
+void rb_mmtk_pin_array_buffer(VALUE array, volatile VALUE *stack_slot);
+
 // MMTk-specific Ruby module (GC::MMTk)
 void rb_mmtk_define_gc_mmtk_module(void);
 VALUE rb_mmtk_plan_name(VALUE _);

mmtk_support.c

@@ -1060,6 +1060,24 @@ rb_mmtk_objbuf_to_elems(rb_mmtk_objbuf_t* objbuf)
  return objbuf->ary;
 }
 
+////////////////////////////////////////////////////////////////////////////////
+// Object pinning
+////////////////////////////////////////////////////////////////////////////////
+
+// Temporarily pin the buffer of an array so that it can be passed to native functions which are
+// not aware of object movement.
+void
+rb_mmtk_pin_array_buffer(VALUE array, volatile VALUE *stack_slot)
+{
+ // We store the reference into the given stack slot so that the conservative stack scanner can
+ // pick it up.
+ if (ARY_EMBED_P(array)) {
+ *stack_slot = array;
+ } else {
+ *stack_slot = RARRAY_EXT(array)->objbuf;
+ }
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 // MMTk-specific Ruby module (GC::MMTk)
 ////////////////////////////////////////////////////////////////////////////////