Add a sticky "PF_DEAD" task flag to keep track of dead processes.

Use this to simplify 'finish_task_switch', but perhaps more
importantly we can use this to track down why some processes
seem to sometimes not die properly even after having been
marked as ZOMBIE. The "task->state" flags are too fluid to 
allow that well.

include/linux/sched.h

@@ -483,6 +483,7 @@ do { if (atomic_dec_and_test(&(tsk)->usage)) __put_task_struct(tsk); } while(0)
 					/* Not implemented yet, only for 486*/
 #define PF_STARTING	0x00000002	/* being created */
 #define PF_EXITING	0x00000004	/* getting shut down */
+#define PF_DEAD		0x00000008	/* Dead */
 #define PF_FORKNOEXEC	0x00000040	/* forked but didn't exec */
 #define PF_SUPERPRIV	0x00000100	/* used super-user privileges */
 #define PF_DUMPCORE	0x00000200	/* dumped core */

kernel/exit.c

@@ -688,6 +688,7 @@ static void exit_notify(struct task_struct *tsk)
 	}
 
 	tsk->state = TASK_ZOMBIE;
+	tsk->flags |= PF_DEAD;
 	/*
 	 * In the preemption case it must be impossible for the task
 	 * to get runnable again, so use "_raw_" unlock to keep

kernel/sched.c

@@ -742,7 +742,7 @@ static inline void finish_task_switch(task_t *prev)
 {
 	runqueue_t *rq = this_rq();
 	struct mm_struct *mm = rq->prev_mm;
-	int drop_task_ref;
+	unsigned long prev_task_flags;
 
 	rq->prev_mm = NULL;
 
@@ -757,14 +757,11 @@ static inline void finish_task_switch(task_t *prev)
 	 * be dropped twice.
 	 * 		Manfred Spraul <manfred@colorfullife.com>
 	 */
-	drop_task_ref = 0;
-	if (unlikely(prev->state & (TASK_DEAD | TASK_ZOMBIE)))
-		drop_task_ref = 1;
-
+	prev_task_flags = prev->flags;
 	finish_arch_switch(rq, prev);
 	if (mm)
 		mmdrop(mm);
-	if (drop_task_ref)
+	if (unlikely(prev_task_flags & PF_DEAD))
 		put_task_struct(prev);
 }