[PATCH] revert "Drop tasklist lock in do_sched_setscheduler"

sched_setscheduler() looks at ->signal->rlim[].  It is unsafe do
dereference ->signal unless tasklist_lock or ->siglock is held (or p ==
current).  We pin the task structure, but this can't prevent from
release_task()->__exit_signal() which sets ->signal = NULL.

Restore tasklist_lock across the setscheduler call.
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Greg KH <greg@kroah.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

kernel/sched.c

@@ -4162,10 +4162,8 @@ do_sched_setscheduler(pid_t pid, int policy, struct sched_param __user *param)
 		read_unlock_irq(&tasklist_lock);
 		return -ESRCH;
 	}
-	get_task_struct(p);
-	read_unlock_irq(&tasklist_lock);
 	retval = sched_setscheduler(p, policy, &lparam);
-	put_task_struct(p);
+	read_unlock_irq(&tasklist_lock);
 
 	return retval;
 }