[PATCH] Thread exit notification by futex

This updates the CLONE_CLEARTID case to use futexes to make it easier
to wait for a thread exit.

glibc/pthreads had been updated to use the TID-futex, this removes an
extra system-call and it also simplifies the pthread_join() code.  The
pthreads testcode works just fine with the new kernel and does not work
with a kernel that does not do the futex wakeup, so it's working fine.

include/linux/futex.h

@@ -6,4 +6,6 @@
 #define FUTEX_WAKE (1)
 #define FUTEX_FD (2)
 
+extern asmlinkage int sys_futex(void *uaddr, int op, int val, struct timespec *utime);
+
 #endif

kernel/fork.c

@@ -26,6 +26,7 @@
 #include <linux/mman.h>
 #include <linux/fs.h>
 #include <linux/security.h>
+#include <linux/futex.h>
 
 #include <asm/pgtable.h>
 #include <asm/pgalloc.h>
@@ -370,12 +371,14 @@ void mm_release(void)
 		tsk->vfork_done = NULL;
 		complete(vfork_done);
 	}
-	if (tsk->user_tid)
+	if (tsk->user_tid) {
 		/*
 		 * We dont check the error code - if userspace has
 		 * not set up a proper pointer then tough luck.
 		 */
 		put_user(0UL, tsk->user_tid);
+		sys_futex(tsk->user_tid, FUTEX_WAKE, 1, NULL);
+	}
 }
 
 static int copy_mm(unsigned long clone_flags, struct task_struct * tsk)