Help with learning scheduler O(1)

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 ? a reference to the code that study - http://lxr.free-electrons.com/source/kernel/sched.c?v=2.6.36 what I learn on line 4682 to 4695. 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.