diff --git a/winsup/cygwin/ChangeLog b/winsup/cygwin/ChangeLog
index c87f6016e..264507fcf 100644
--- a/winsup/cygwin/ChangeLog
+++ b/winsup/cygwin/ChangeLog
@@ -1,3 +1,9 @@
+2012-05-23  Corinna Vinschen  <corinna@vinschen.de>
+
+	* fhandler.h (refcnt): Add i interlocked.  Explain why.
+	* winbase.h (ilockadd): New function.
+	(InterlockedAdd): Define as ilockadd.
+
 2012-05-22  Corinna Vinschen  <corinna@vinschen.de>
 
 	* devices.in: Fix native name of /dev/kmem.
diff --git a/winsup/cygwin/fhandler.h b/winsup/cygwin/fhandler.h
index 18e829aa6..84f6e4c3f 100644
--- a/winsup/cygwin/fhandler.h
+++ b/winsup/cygwin/fhandler.h
@@ -185,7 +185,11 @@ class fhandler_base
   long refcnt(long i = 0)
   {
     debug_only_printf ("%p, %s, i %d, refcnt %ld", this, get_name (), i, _refcnt + i);
-    return _refcnt += i;
+    /* This MUST be an interlocked operation.  If multiple threads access the
+       same descriptor in quick succession, a context switch can interrupt
+       the += operation and we wrongly end up with a refcnt of 0 in the
+       cygheap_fdget destructor. */
+    return i ? InterlockedAdd (&_refcnt, i) : _refcnt;
   }
   class fhandler_base *archetype;
   int usecount;
diff --git a/winsup/cygwin/winbase.h b/winsup/cygwin/winbase.h
index 00c71a923..4d92a9261 100644
--- a/winsup/cygwin/winbase.h
+++ b/winsup/cygwin/winbase.h
@@ -1,6 +1,6 @@
 /* winbase.h
 
-   Copyright 2002, 2003, 2004, 2008 Red Hat, Inc.
+   Copyright 2002, 2003, 2004, 2008, 2012 Red Hat, Inc.
 
 This software is a copyrighted work licensed under the terms of the
 Cygwin license.  Please consult the file "CYGWIN_LICENSE" for
@@ -11,6 +11,21 @@ details. */
 #ifndef _WINBASE2_H
 #define _WINBASE2_H
 
+/* For some unknown reason, InterlockedAdd is only supported on Itanium
+   when using the Windows headers.  Fortunately we're not restricted to the
+   Windows headers :) */
+extern __inline__ long
+ilockadd (volatile long *m, long value)
+{
+  register int __res;
+  __asm__ __volatile__ ("\n\
+	movl	%3,%0\n\
+	lock	xadd %0,%1\n\
+	addl	%3,%0\n\
+	": "=&r" (__res), "=m" (*m): "m" (*m), "r" (value): "cc");
+  return __res;
+}
+
 extern __inline__ long
 ilockincr (volatile long *m)
 {
@@ -65,6 +80,8 @@ ilockcmpexch (volatile long *t, long v, long c)
   });
 }
 
+#undef InterlockedAdd
+#define InterlockedAdd ilockadd
 #undef InterlockedIncrement
 #define InterlockedIncrement ilockincr
 #undef InterlockedDecrement