freezer: remove racy clear_freeze_flag() and set PF_NOFREEZE on dead tasks

clear_freeze_flag() in exit_mm() is racy.  Freezing can start
afterwards.  Remove it.  Skipping freezer for exiting task will be
properly implemented later.

Also, freezable() was testing exit_state directly to make system
freezer ignore dead tasks.  Let the exiting task set PF_NOFREEZE after
entering TASK_DEAD instead.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>

kernel/exit.c

@@ -679,8 +679,6 @@ static void exit_mm(struct task_struct * tsk)
 	tsk->mm = NULL;
 	up_read(&mm->mmap_sem);
 	enter_lazy_tlb(mm, current);
-	/* We don't want this task to be frozen prematurely */
-	clear_freeze_flag(tsk);
 	task_unlock(tsk);
 	mm_update_next_owner(mm);
 	mmput(mm);
@@ -1040,6 +1038,7 @@ NORET_TYPE void do_exit(long code)
 	exit_rcu();
 	/* causes final put_task_struct in finish_task_switch(). */
 	tsk->state = TASK_DEAD;
+	tsk->flags |= PF_NOFREEZE;	/* tell freezer to ignore us */
 	schedule();
 	BUG();
 	/* Avoid "noreturn function does return".  */

kernel/power/process.c

@@ -25,8 +25,7 @@
 static inline int freezable(struct task_struct * p)
 {
 	if ((p == current) ||
-	    (p->flags & PF_NOFREEZE) ||
-	    (p->exit_state != 0))
+	    (p->flags & PF_NOFREEZE))
 		return 0;
 	return 1;
 }