[PATCH] sched: arch preempt notifier mechanism

This adds a general mechanism whereby a task can request the scheduler to
notify it whenever it is preempted or scheduled back in.  This allows the
task to swap any special-purpose registers like the fpu or Intel's VT
registers.

Signed-off-by: Avi Kivity <avi@qumranet.com>
[ mingo@elte.hu: fixes, cleanups ]
Signed-off-by: Ingo Molnar <mingo@elte.hu>

include/linux/sched.h

@@ -935,6 +935,11 @@ struct task_struct {
 	struct sched_class *sched_class;
 	struct sched_entity se;
 
+#ifdef CONFIG_PREEMPT_NOTIFIERS
+	/* list of struct preempt_notifier: */
+	struct hlist_head preempt_notifiers;
+#endif
+
 	unsigned short ioprio;
 #ifdef CONFIG_BLK_DEV_IO_TRACE
 	unsigned int btrace_seq;