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`).
mmtk · Sep 22, 2023 · d82b82e · d82b82e
1 parent c56f215
commit d82b82e
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diff --git a/include/ruby/internal/core/rarray.h b/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

diff --git a/internal/mmtk_support.h b/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 _);

diff --git a/mmtk_support.c b/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)
 ////////////////////////////////////////////////////////////////////////////////