Revert https://github.com/ruby/ruby/pull/2486

This reverts commits: 10d6a3aca7 8ba48c1b85 fba8627dc1 dd883de5ba
6c6a25feca 167e6b48f1 7cb96d41a5 3207979278 595b3c4fdd 1521f7cf89
c11c5e69ac cf33608203 3632a812c0 f56506be0d 86427a3219 .

The reason for the revert is that we observe ABA problem around
inline method cache.  When a cache misshits, we search for a
method entry.  And if the entry is identical to what was cached
before, we reuse the cache.  But the commits we are reverting here
introduced situations where a method entry is freed, then the
identical memory region is used for another method entry.  An
inline method cache cannot detect that ABA.

Here is a code that reproduce such situation:

```ruby
require 'prime'

class << Integer
  alias org_sqrt sqrt
  def sqrt(n)
    raise
  end

  GC.stress = true
  Prime.each(7*37){} rescue nil # <- Here we populate CC
  class << Object.new; end

  # These adjacent remove-then-alias maneuver
  # frees a method entry, then immediately
  # reuses it for another.
  remove_method :sqrt
  alias sqrt org_sqrt
end

Prime.each(7*37).to_a # <- SEGV
```

1 files changed, 12 insertions, 1 deletions

diff --git a/vm.c b/vm.c
index 16a15c2151..e104688d71 100644
--- a/vm.c
+++ b/vm.c
@@ -298,6 +298,17 @@ rb_vm_cref_new_toplevel(void)
     return vm_cref_new_toplevel(GET_EC());
 }
 
+static void
+vm_cref_dump(const char *mesg, const rb_cref_t *cref)
+{
+    fprintf(stderr, "vm_cref_dump: %s (%p)\n", mesg, (void *)cref);
+
+    while (cref) {
+	fprintf(stderr, "= cref| klass: %s\n", RSTRING_PTR(rb_class_path(CREF_CLASS(cref))));
+	cref = CREF_NEXT(cref);
+    }
+}
+
 void
 rb_vm_block_ep_update(VALUE obj, const struct rb_block *dst, const VALUE *ep)
 {
@@ -1595,7 +1606,7 @@ static enum rb_id_table_iterator_result
 check_redefined_method(ID mid, VALUE value, void *data)
 {
     VALUE klass = (VALUE)data;
-    const rb_method_entry_t *me = (const rb_method_entry_t *)value;
+    const rb_method_entry_t *me = (rb_method_entry_t *)value;
     const rb_method_entry_t *newme = rb_method_entry(klass, mid);
 
     if (newme != me) rb_vm_check_redefinition_opt_method(me, me->owner);