ia64: Define new "arch_switch" macros so that the tasklist_lock can

	be released during the (low-level) context-switch.  Patch by
	Erich Focht.

include/asm-ia64/system.h

@@ -432,6 +432,38 @@ extern void ia64_load_extra (struct task_struct *task);
 } while (0)
 #endif
 
+/*
+ * On IA-64, we don't want to hold the runqueue's lock during the low-level context-switch,
+ * because that could cause a deadlock.  Here is an example by Erich Focht:
+ *
+ * Example:
+ * CPU#0:
+ * schedule()
+ *    -> spin_lock_irq(&rq->lock)
+ *    -> context_switch()
+ *       -> wrap_mmu_context()
+ *          -> read_lock(&tasklist_lock)
+ *
+ * CPU#1:
+ * sys_wait4() or release_task() or forget_original_parent()
+ *    -> write_lock(&tasklist_lock)
+ *    -> do_notify_parent()
+ *       -> wake_up_parent()
+ *          -> try_to_wake_up()
+ *             -> spin_lock_irq(&parent_rq->lock)
+ *
+ * If the parent's rq happens to be on CPU#0, we'll wait for the rq->lock
+ * of that CPU which will not be released, because there we wait for the
+ * tasklist_lock to become available.
+ */
+#define prepare_arch_switch(rq, next)		\
+do {						\
+	spin_lock(&(next)->switch_lock);	\
+	spin_unlock(&(rq)->lock);		\
+} while (0)
+#define finish_arch_switch(rq, prev)	spin_unlock_irq(&(prev)->switch_lock)
+#define task_running(rq, p) 		((rq)->curr == (p) || spin_is_locked(&(p)->switch_lock))
+
 #endif /* __KERNEL__ */
 
 #endif /* __ASSEMBLY__ */