question scheduler

Hello all continue to study the scheduler is O(1 ) there was one difficulty with the string 2 on 3, in that these blocking occurs after lines ? 1   rcu_read_lock(); 2   pcred = __task_cred(p); 3   match = (cred->euid == pcred->euid ||                      cred->euid == pcread->uid); 4   rcu_read_unlock(); I ask because in the comments there is not described . Thank.