Fix long standing set_rt_params race by disallowing it.

There really is no reason why RT params of a RT task should be changed. The
sporadic task model does not allow for it, the schedulers don't expect it,
and adaptive tasks will be implemented differently.

1 files changed, 12 insertions, 1 deletions

diff --git a/kernel/litmus.c b/kernel/litmus.c
index 6b5022ea94..2c545092b8 100644
--- a/kernel/litmus.c
+++ b/kernel/litmus.c
@@ -70,7 +70,9 @@ asmlinkage long sys_set_rt_mode(int newmode)
  *         EINVAL  if either period or execution cost is <=0
  *         0       if success
  *
- *  FIXME: This code is racy during real-time mode.
+ * Only non-real-time tasks may be configured with this system call 
+ * to avoid races with the scheduler. In practice, this means that a
+ * task's parameters must be set _before_ calling sys_prepare_rt_task()
  */
 asmlinkage long sys_set_rt_task_param(pid_t pid, rt_param_t __user * param)
 {
@@ -93,7 +95,16 @@ asmlinkage long sys_set_rt_task_param(pid_t pid, rt_param_t __user * param)
         if (!(target = find_task_by_pid(pid))) {
                 retval = -ESRCH;
                 goto out_unlock;
+        }       
+
+        if (is_realtime(target)) {
+                /* The task is already a real-time task.
+                 * We cannot not allow parameter changes at this point.
+                 */
+                retval = -EPERM;
+                goto out_unlock;
         }
+
         if (tp.exec_cost <= 0)
                 goto out_unlock;
         if (tp.period <= 0)