33 Concurrency support library [thread]

33.11 Safe reclamation [saferecl]

33.11.2 Read-copy update (RCU) [saferecl.rcu] General [saferecl.rcu.general]

RCU is a synchronization mechanism that can be used for linked data structures that are frequently read, but seldom updated.
RCU does not provide mutual exclusion, but instead allows the user to schedule specified actions such as deletion at some later time.
A class type T is rcu-protectable if it has exactly one base class of type rcu_obj_base<T, D> for some D, and that base is public and non-virtual, and it has no base classes of type rcu_obj_base<X, Y> for any other combination X, Y.
An object is rcu-protectable if it is of rcu-protectable type.
An invocation of unlock U on an rcu_domain dom corresponds to an invocation of lock L on dom if L is sequenced before U and either
  • no other invocation of lock on dom is sequenced after L and before U, or
  • every invocation of unlock U2 on dom such that L is sequenced before U2 and U2 is sequenced before U corresponds to an invocation of lock L2 on dom such that L is sequenced before L2 and L2 is sequenced before U2.
[Note 1: 
This pairs nested locks and unlocks on a given domain in each thread.
— end note]
A region of RCU protection on a domain dom starts with a lock L on dom and ends with its corresponding unlock U.
Given a region of RCU protection R on a domain dom and given an evaluation E that scheduled another evaluation F in dom, if E does not strongly happen before the start of R, the end of R strongly happens before evaluating F.
The evaluation of a scheduled evaluation is potentially concurrent with any other scheduled evaluation.
Each scheduled evaluation is evaluated at most once.