Help with learning scheduler O(1)

[Meyer Lansky]

 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.
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