diff -r 5841dc1964f0 -r a97fd181b813 src/share/vm/oops/methodOop.cpp
--- a/src/share/vm/oops/methodOop.cpp	Tue Feb 22 15:26:36 2011 -0800
+++ b/src/share/vm/oops/methodOop.cpp	Wed Feb 23 11:18:16 2011 -0800
@@ -985,9 +985,11 @@
                                               IsUnsafeConc,
                                               CHECK_(methodHandle()));
   methodHandle newm (THREAD, newm_oop);
+  NOT_PRODUCT(int nmsz = newm->is_parsable() ? newm->size() : -1;)
   int new_method_size = newm->method_size();
   // Create a shallow copy of methodOopDesc part, but be careful to preserve the new constMethodOop
   constMethodOop newcm = newm->constMethod();
+  NOT_PRODUCT(int ncmsz = newcm->is_parsable() ? newcm->size() : -1;)
   int new_const_method_size = newm->constMethod()->object_size();
 
   memcpy(newm(), m(), sizeof(methodOopDesc));
@@ -999,9 +1001,19 @@
   // or concurrent marking but those phases will be correct.  Setting and
   // resetting is done in preference to a careful copying into newcm to
   // avoid having to know the precise layout of a constMethodOop.
-  m->constMethod()->set_is_conc_safe(false);
+  m->constMethod()->set_is_conc_safe(oopDesc::IsUnsafeConc);
+  assert(m->constMethod()->is_parsable(), "Should remain parsable");
+
+  // NOTE: this is a reachable object that transiently signals "conc_unsafe"
+  // However, no allocations are done during this window
+  // during which it is tagged conc_unsafe, so we are assured that any concurrent
+  // thread will not wait forever for the object to revert to "conc_safe".
+  // Further, any such conc_unsafe object will indicate a stable size
+  // through the transition.
   memcpy(newcm, m->constMethod(), sizeof(constMethodOopDesc));
-  m->constMethod()->set_is_conc_safe(true);
+  m->constMethod()->set_is_conc_safe(oopDesc::IsSafeConc);
+  assert(m->constMethod()->is_parsable(), "Should remain parsable");
+
   // Reset correct method/const method, method size, and parameter info
   newcm->set_method(newm());
   newm->set_constMethod(newcm);
@@ -1035,6 +1047,8 @@
 
   // Only set is_conc_safe to true when changes to newcm are
   // complete.
+  assert(!newm->is_parsable()  || nmsz  < 0 || newm->size()  == nmsz,  "newm->size()  inconsistency");
+  assert(!newcm->is_parsable() || ncmsz < 0 || newcm->size() == ncmsz, "newcm->size() inconsistency");
   newcm->set_is_conc_safe(true);
   return newm;
 }
@@ -1372,7 +1386,7 @@
 }
 
 // See comment in methodOop.hpp which explains why this exists.
-#if defined(_M_AMD64) && MSC_VER >= 1400
+#if defined(_M_AMD64) && _MSC_VER >= 1400
 #pragma optimize("", off)
 void CompressedLineNumberWriteStream::write_pair(int bci, int line) {
   write_pair_inline(bci, line);