sched: Fix fork() error path to not crash

In dup_task_struct(), if arch_dup_task_struct() fails, the clean up
code fails to clean up correctly.  That's because the clean up
code depends on unininitalized ti->task pointer.  We fix this
by making sure that the task and thread_info know about each other
before we attempt to take the error path.

Signed-off-by: Salman Qazi <sqazi@google.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/20120626011815.11323.5533.stgit@dungbeetle.mtv.corp.google.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/fork.c

@@ -304,12 +304,17 @@ static struct task_struct *dup_task_struct(struct task_struct *orig)
 	}
 
 	err = arch_dup_task_struct(tsk, orig);
-	if (err)
-		goto out;
 
+	/*
+	 * We defer looking at err, because we will need this setup
+	 * for the clean up path to work correctly.
+	 */
 	tsk->stack = ti;
-
 	setup_thread_stack(tsk, orig);
+
+	if (err)
+		goto out;
+
 	clear_user_return_notifier(tsk);
 	clear_tsk_need_resched(tsk);
 	stackend = end_of_stack(tsk);