[saferecl.rcu] # 32 Concurrency support library [[thread]](./#thread) ## 32.11 Safe reclamation [[saferecl]](saferecl#rcu) ### 32.11.2 Read-copy update (RCU) [saferecl.rcu] #### [32.11.2.1](#general) General [[saferecl.rcu.general]](saferecl.rcu.general) [1](#general-1) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L12920) RCU is a synchronization mechanism that can be used for linked data structures that are frequently read, but seldom updated[.](#general-1.sentence-1) RCU does not provide mutual exclusion, but instead allows the user to schedule specified actions such as deletion at some later time[.](#general-1.sentence-2) [2](#general-2) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L12928) A class type T is [*rcu-protectable*](#def:rcu-protectable "32.11.2.1 General [saferecl.rcu.general]") if it has exactly one base class of type rcu_obj_base for some D, and that base is public and non-virtual, and it has no base classes of type rcu_obj_base for any other combination X, Y[.](#general-2.sentence-1) An object is rcu-protectable if it is of rcu-protectable type[.](#general-2.sentence-2) [3](#general-3) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L12936) 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 - [(3.1)](#general-3.1) no other invocation of lock on dom is sequenced after L and before U, or - [(3.2)](#general-3.2) 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[.](#general-3.sentence-1) [*Note [1](#general-note-1)*: This pairs nested locks and unlocks on a given domain in each thread[.](#general-3.sentence-2) — *end note*] [4](#general-4) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L12954) A [*region of RCU protection*](#def:region_of_RCU_protection "32.11.2.1 General [saferecl.rcu.general]") on a domain dom starts with a lock L on dom and ends with its corresponding unlock U[.](#general-4.sentence-1) [5](#general-5) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L12959) 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[.](#general-5.sentence-1) [6](#general-6) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L12965) The evaluation of a scheduled evaluation is potentially concurrent with any other scheduled evaluation[.](#general-6.sentence-1) Each scheduled evaluation is evaluated at most once[.](#general-6.sentence-2) #### [32.11.2.2](#rcu.syn) Header synopsis [[rcu.syn]](rcu.syn) [🔗](#header:%3crcu%3e) namespace std {// [[saferecl.rcu.base]](#base "32.11.2.3 Class template rcu_­obj_­base"), class template rcu_obj_basetemplate> class rcu_obj_base; // [[saferecl.rcu.domain]](#domain "32.11.2.4 Class rcu_­domain"), class rcu_domainclass rcu_domain; // [[saferecl.rcu.domain.func]](#domain.func "32.11.2.4.3 Non-member functions"), non-member functions rcu_domain& rcu_default_domain() noexcept; void rcu_synchronize(rcu_domain& dom = rcu_default_domain()) noexcept; void rcu_barrier(rcu_domain& dom = rcu_default_domain()) noexcept; template>void rcu_retire(T* p, D d = D(), rcu_domain& dom = rcu_default_domain());} #### [32.11.2.3](#base) Class template rcu_obj_base [[saferecl.rcu.base]](saferecl.rcu.base) [1](#base-1) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L12992) Objects of type T to be protected by RCU inherit from a specialization rcu_obj_base for some D[.](#base-1.sentence-1) namespace std {template>class rcu_obj_base {public:void retire(D d = D(), rcu_domain& dom = rcu_default_domain()) noexcept; protected: rcu_obj_base() = default; rcu_obj_base(const rcu_obj_base&) = default; rcu_obj_base(rcu_obj_base&&) = default; rcu_obj_base& operator=(const rcu_obj_base&) = default; rcu_obj_base& operator=(rcu_obj_base&&) = default; ~rcu_obj_base() = default; private: D *deleter*; // *exposition only*};} [2](#base-2) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13015) The behavior of a program that adds specializations for rcu_obj_base is undefined[.](#base-2.sentence-1) [3](#base-3) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13019) T may be an incomplete type[.](#base-3.sentence-1) It shall be complete before any member of the resulting specialization ofrcu_obj_base is referenced[.](#base-3.sentence-2) [4](#base-4) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13024) D shall be a function object type ([[function.objects]](function.objects "22.10 Function objects")) for which, given a value d of type D and a value ptr of type T*, the expression d(ptr) is valid[.](#base-4.sentence-1) [5](#base-5) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13031) D shall meet the requirements for[*Cpp17DefaultConstructible*](utility.arg.requirements#:Cpp17DefaultConstructible "16.4.4.2 Template argument requirements [utility.arg.requirements]") and [*Cpp17MoveAssignable*](utility.arg.requirements#:Cpp17MoveAssignable "16.4.4.2 Template argument requirements [utility.arg.requirements]")[.](#base-5.sentence-1) [6](#base-6) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13035) If D is trivially copyable, all specializations of rcu_obj_base are trivially copyable[.](#base-6.sentence-1) [🔗](#base-itemdecl:1) `void retire(D d = D(), rcu_domain& dom = rcu_default_domain()) noexcept; ` [7](#base-7) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13044) *Mandates*: T is an rcu-protectable type[.](#base-7.sentence-1) [8](#base-8) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13048) *Preconditions*: *this is a base class subobject of an object x of type T[.](#base-8.sentence-1) The member function rcu_obj_base​::​retire was not invoked on x before[.](#base-8.sentence-2) The assignment to *deleter* does not exit via an exception[.](#base-8.sentence-3) [9](#base-9) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13056) *Effects*: Evaluates *deleter* = std​::​move(d) and schedules the evaluation of the expression *deleter*( addressof(x)) in the domain dom; the behavior is undefined if that evaluation exits via an exception[.](#base-9.sentence-1) May invoke scheduled evaluations in dom[.](#base-9.sentence-2) [*Note [1](#base-note-1)*: If such evaluations acquire resources held across any invocation ofretire on dom, deadlock can occur[.](#base-9.sentence-3) — *end note*] #### [32.11.2.4](#domain) Class rcu_domain [[saferecl.rcu.domain]](saferecl.rcu.domain) #### [32.11.2.4.1](#domain.general) General [[saferecl.rcu.domain.general]](saferecl.rcu.domain.general) namespace std {class rcu_domain {public: rcu_domain(const rcu_domain&) = delete; rcu_domain& operator=(const rcu_domain&) = delete; void lock() noexcept; bool try_lock() noexcept; void unlock() noexcept; };} [1](#domain.general-1) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13089) This class meets the requirements of[*Cpp17Lockable*](thread.req.lockable.req#:Cpp17Lockable "32.2.5.3 Cpp17Lockable requirements [thread.req.lockable.req]") ([[thread.req.lockable.req]](thread.req.lockable.req "32.2.5.3 Cpp17Lockable requirements")) and provides regions of RCU protection[.](#domain.general-1.sentence-1) [*Example [1](#domain.general-example-1)*: std::scoped_lock rlock(rcu_default_domain()); — *end example*] [2](#domain.general-2) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13099) The functions lock and unlock establish (possibly nested) regions of RCU protection[.](#domain.general-2.sentence-1) #### [32.11.2.4.2](#domain.members) Member functions [[saferecl.rcu.domain.members]](saferecl.rcu.domain.members) [🔗](#lib:lock,rcu_domain) `void lock() noexcept; ` [1](#domain.members-1) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13111) *Effects*: Opens a region of RCU protection[.](#domain.members-1.sentence-1) [2](#domain.members-2) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13115) *Remarks*: Calls to lock do not introduce a data race ([[intro.races]](intro.races "6.10.2.2 Data races")) involving *this[.](#domain.members-2.sentence-1) [🔗](#lib:try_lock,rcu_domain) `bool try_lock() noexcept; ` [3](#domain.members-3) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13127) *Effects*: Equivalent to lock()[.](#domain.members-3.sentence-1) [4](#domain.members-4) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13131) *Returns*: true[.](#domain.members-4.sentence-1) [🔗](#lib:unlock,rcu_domain) `void unlock() noexcept; ` [5](#domain.members-5) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13142) *Preconditions*: A call to lock that opened an unclosed region of RCU protection is sequenced before the call to unlock[.](#domain.members-5.sentence-1) [6](#domain.members-6) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13148) *Effects*: Closes the unclosed region of RCU protection that was most recently opened[.](#domain.members-6.sentence-1) May invoke scheduled evaluations in *this[.](#domain.members-6.sentence-2) [7](#domain.members-7) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13154) [*Note [1](#domain.members-note-1)*: If such evaluations acquire resources held across any invocation of unlock on *this, deadlock can occur[.](#domain.members-7.sentence-1) — *end note*] [8](#domain.members-8) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13161) *Remarks*: Calls to unlock do not introduce a data race involving *this[.](#domain.members-8.sentence-1) [*Note [2](#domain.members-note-2)*: Evaluation of scheduled evaluations can still cause a data race[.](#domain.members-8.sentence-2) — *end note*] #### [32.11.2.4.3](#domain.func) Non-member functions [[saferecl.rcu.domain.func]](saferecl.rcu.domain.func) [🔗](#lib:rcu_default_domain) `rcu_domain& rcu_default_domain() noexcept; ` [1](#domain.func-1) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13177) *Returns*: A reference to a static-duration object of type rcu_domain[.](#domain.func-1.sentence-1) A reference to the same object is returned every time this function is called[.](#domain.func-1.sentence-2) [🔗](#lib:rcu_synchronize) `void rcu_synchronize(rcu_domain& dom = rcu_default_domain()) noexcept; ` [2](#domain.func-2) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13189) *Effects*: If the call to rcu_synchronize does not strongly happen before the lock opening an RCU protection region R on dom, blocks until the unlock closing R happens[.](#domain.func-2.sentence-1) [3](#domain.func-3) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13195) *Synchronization*: The unlock closing R strongly happens before the return from rcu_synchronize[.](#domain.func-3.sentence-1) [🔗](#lib:rcu_barrier) `void rcu_barrier(rcu_domain& dom = rcu_default_domain()) noexcept; ` [4](#domain.func-4) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13207) *Effects*: May evaluate any scheduled evaluations in dom[.](#domain.func-4.sentence-1) For any evaluation that happens before the call to rcu_barrier and that schedules an evaluation E in dom, blocks until E has been evaluated[.](#domain.func-4.sentence-2) [5](#domain.func-5) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13214) *Synchronization*: The evaluation of any such E strongly happens before the return from rcu_barrier[.](#domain.func-5.sentence-1) [*Note [1](#domain.func-note-1)*: A call to rcu_barrier does not imply a call to rcu_synchronize and vice versa[.](#domain.func-5.sentence-2) — *end note*] [🔗](#lib:rcu_retire) `template> void rcu_retire(T* p, D d = D(), rcu_domain& dom = rcu_default_domain()); ` [6](#domain.func-6) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13232) *Mandates*: is_move_constructible_v is true and the expression d(p) is well-formed[.](#domain.func-6.sentence-1) [7](#domain.func-7) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13237) *Preconditions*: D meets the [*Cpp17MoveConstructible*](utility.arg.requirements#:Cpp17MoveConstructible "16.4.4.2 Template argument requirements [utility.arg.requirements]") and[*Cpp17Destructible*](utility.arg.requirements#:Cpp17Destructible "16.4.4.2 Template argument requirements [utility.arg.requirements]") requirements[.](#domain.func-7.sentence-1) [8](#domain.func-8) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13242) *Effects*: May allocate memory[.](#domain.func-8.sentence-1) It is unspecified whether the memory allocation is performed by invoking operator new[.](#domain.func-8.sentence-2) Initializes an object d1 of type D from std​::​move(d)[.](#domain.func-8.sentence-3) Schedules the evaluation of d1(p) in the domain dom; the behavior is undefined if that evaluation exits via an exception[.](#domain.func-8.sentence-4) May invoke scheduled evaluations in dom[.](#domain.func-8.sentence-5) [*Note [2](#domain.func-note-2)*: If rcu_retire exits via an exception, no evaluation is scheduled[.](#domain.func-8.sentence-6) — *end note*] [9](#domain.func-9) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13256) *Throws*: bad_alloc or any exception thrown by the initialization of d1[.](#domain.func-9.sentence-1) [10](#domain.func-10) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13260) [*Note [3](#domain.func-note-3)*: If scheduled evaluations acquire resources held across any invocation of rcu_retire on dom, deadlock can occur[.](#domain.func-10.sentence-1) — *end note*]