2 This file is a part of libcds - Concurrent Data Structures library
4 (C) Copyright Maxim Khizhinsky (libcds.dev@gmail.com) 2006-2016
6 Source code repo: http://github.com/khizmax/libcds/
7 Download: http://sourceforge.net/projects/libcds/files/
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31 #ifndef CDSLIB_URCU_DETAILS_BASE_H
32 #define CDSLIB_URCU_DETAILS_BASE_H
34 #include <cds/algo/atomic.h>
35 #include <cds/gc/details/retired_ptr.h>
36 #include <cds/details/allocator.h>
37 #include <cds/os/thread.h>
38 #include <cds/details/marked_ptr.h>
41 /// User-space Read-Copy Update (URCU) namespace
42 /** @ingroup cds_garbage_collector
45 This namespace contains declarations for different types of Read-Copy Update (%RCU)
46 synchronization primitive and data structures developed for RCU.
47 In <b>libcds</b> %RCU is used as garbage collector.
50 - [2009] M.Desnoyers "Low-Impact Operating System Tracing" PhD Thesis,
51 Chapter 6 "User-Level Implementations of Read-Copy Update"
52 - [2011] M.Desnoyers, P.McKenney, A.Stern, M.Dagenias, J.Walpole "User-Level
53 Implementations of Read-Copy Update"
55 <b>Informal introduction to user-space %RCU</b>
57 [<i>From Desnoyer's papers</i>] %RCU is a synchronization mechanism that was added to the
58 Linux kernel in October of 2002. %RCU achieves scalability improvements by allowing
59 reads to occur concurrently with updates. In contrast to conventional locking
60 primitives that ensure mutual exclusion among concurrent threads regardless of whether
61 they be readers or updaters, or with reader-writer locks that allow concurrent reads
62 but not in the presence of updates, %RCU supports concurrency between multiple updaters
63 and multiple readers. %RCU ensures that data are not freed up until all pre-existing
64 critical sections complete. %RCU defines and uses efficient and scalable mechanisms
65 for deferring reclamation of old data. These mechanisms distribute the work among read and update
66 paths in such a way as to make read paths extremely fast.
68 %RCU readers execute within %RCU read-side critical sections. Each such critical section begins with
69 \p rcu_read_lock(), ends with \p rcu_read_unlock() (in \p libcds these primitives are the methods of
70 GC class and are usually called \p access_lock and \p access_unlock respectively). Read-side
71 critical sections can be nested.
72 The performance benefits of %RCU are due to the fact that \p rcu_read_lock()
73 and \p rcu_read_unlock() are exceedingly fast.
75 When a thread is not in an %RCU read-side critical section, it is in a quiescent state.
76 A quiescent state that persists for a significant time period is an extended quiescent state.
77 Any time period during which every thread has been in at least one quiescent state
78 is a grace period; this implies that every %RCU read-side critical section
79 that starts before a grace period must end before that grace period does.
80 Distinct grace periods may overlap, either partially or completely. Any time period
81 that includes a grace period is by definition itself a grace period.
82 Each grace period is guaranteed to complete as long as all read-side critical sections
83 are finite in duration; thus even a constant flow of such critical sections is unable to
84 extend an %RCU grace period indefinitely.
86 Suppose that readers enclose each of their data-structure traversals in
87 an %RCU read-side critical section. If an updater first removes an element
88 from such a data structure and then waits for a grace period, there can be
89 no more readers accessing that element. The updater can then carry out destructive
90 operations, for example freeing the element, without disturbing any readers.
92 The %RCU update is split into two phases, a removal phase and a reclamation phase.
93 These two phases must be separated by a grace period, for example via the \p synchronize_rcu()
94 primitive, which initiates a grace period and waits for it to finish.
95 During the removal phase, the %RCU update removes elements from a shared data structure.
96 The removed data elements will only be accessible to read-side critical sections
97 that ran concurrently with the removal phase, which are guaranteed to complete before the
98 grace period ends. Therefore the reclamation phase can safely free the data elements
99 removed by the removal phase.
101 Desnoyers describes several classes of user-space %RCU implementations:
102 - The Quiescent-State-Based Reclamation (QSBR) %RCU implementation offers
103 the best possible read-side performance, but requires that each thread periodically
104 calls a function to announce that it is in a quiescent state, thus strongly
105 constraining the application design. This type of %RCU is not implemented in \p libcds.
106 - The general-purpose %RCU implementation places almost no constraints on the application's
107 design, thus being appropriate for use within a general-purpose library, but it has
108 relatively higher read-side overhead. The \p libcds contains several implementations of general-purpose
109 %RCU: \ref general_instant, \ref general_buffered, \ref general_threaded.
110 - The signal-handling %RCU presents an implementation having low read-side overhead and
111 requiring only that the application give up one POSIX signal to %RCU update processing.
112 The \p libcds contains several implementations if signal-handling %RCU: \ref signal_buffered,
113 \ref signal_threaded.
115 @note The signal-handled %RCU is defined only for UNIX-like systems, not for Windows.
117 @anchor cds_urcu_type
118 <b>RCU implementation type</b>
120 There are several internal implementation of RCU (all declared in \p %cds::urcu namespace):
121 - \ref general_instant - general purpose RCU with immediate reclamation
122 - \ref general_buffered - general purpose RCU with deferred (buffered) reclamation
123 - \ref general_threaded - general purpose RCU with special reclamation thread
124 - \ref signal_buffered - signal-handling RCU with deferred (buffered) reclamation
125 - \ref signal_threaded - signal-handling RCU with special reclamation thread
127 You cannot create an object of any of those classes directly.
128 Instead, you should use wrapper classes.
129 The wrapper simplifies creation and usage of RCU singleton objects
130 and has the reacher interface that combines interfaces of wrapped class i.e. RCU global part like
131 \p synchronize, and corresponding RCU thread-specific interface like \p access_lock, \p access_unlock and \p retire_ptr.
134 There are several wrapper classes (all declared in \p %cds::urcu namespace)
135 - \ref cds_urcu_general_instant_gc "gc<general_instant>" - general purpose RCU with immediate reclamation,
136 include file <tt><cds/urcu/general_instant.h></tt>
137 - \ref cds_urcu_general_buffered_gc "gc<general_buffered>" - general purpose RCU with deferred (buffered) reclamation,
138 include file <tt><cds/urcu/general_buffered.h></tt>
139 - \ref cds_urcu_general_threaded_gc "gc<general_threaded>" - general purpose RCU with special reclamation thread
140 include file <tt><cds/urcu/general_threaded.h></tt>
141 - \ref cds_urcu_signal_buffered_gc "gc<signal_buffered>" - signal-handling RCU with deferred (buffered) reclamation
142 include file <tt><cds/urcu/signal_buffered.h></tt>
143 - \ref cds_urcu_signal_threaded_gc "gc<signal_threaded>" - signal-handling RCU with special reclamation thread
144 include file <tt><cds/urcu/signal_threaded.h></tt>
146 Any RCU-related container in \p libcds expects that its \p RCU template parameter is one of those wrapper.
148 @anchor cds_urcu_tags
149 For simplicity, in some algorithms instead of using RCU implementation type
150 you should specify corresponding RCU tags (all declared in \p %cds::urcu namespace):
151 - \ref general_instant_tag - for \ref general_instant
152 - \ref general_buffered_tag - for \ref general_buffered
153 - \ref general_threaded_tag - for \ref general_threaded
154 - \ref signal_buffered_tag - for \ref signal_buffered
155 - \ref signal_threaded_tag - for \ref signal_threaded
157 @anchor cds_urcu_performance
160 As a result of our experiments we can range above %RCU implementation in such order,
161 from high to low performance:
162 - <tt>gc<general_buffered></tt> - high
163 - <tt>gc<general_threaded></tt>
164 - <tt>gc<signal_buffered></tt>
165 - <tt>gc<signal_threaded></tt>
166 - <tt>gc<general_instant></tt> - low
168 This estimation is very rough and depends on many factors:
169 type of payload - mostly read-only (seeking) or read-write (inserting and deleting), -
170 a hardware, your application, and so on.
172 @anchor cds_urcu_howto
175 Usually, in your application you use only one \ref cds_urcu_gc "type of RCU" that is the best for your needs.
176 However, the library allows to apply several RCU singleton in one application.
177 The only limitation is that only one object of each RCU type can be instantiated.
178 Since each RCU type is a template class the creation of two object of one RCU type class
179 with different template arguments is an error and is not supported.
180 However, it is correct when your RCU objects relates to different RCU types.
182 @note If you want to use \p %signal_buffered and \p %signal_threaded RCU in your application simultaneously,
183 you should specify different signal number for each signal-handled RCU type on construction time,
184 for example, \p SIGUSR1 and \p SIGUSR2 respectively. By default, both signal-handled RCU implementation
185 share \p SIGUSR1 signal and cannot be applied together.
187 In \p libcds, many GC-based ordered list, set and map template classes have %RCU-related specializations
188 that hide the %RCU specific details.
190 RCU GC is initialized in usual way: you should declare an object of type \p cds::urcu::gc< RCU_type >,
193 #include <cds/urcu/general_buffered.h>
195 typedef cds::urcu::gc< cds::urcu::general_buffered<> > rcu_gpb;
201 // Initialize general_buffered RCU
204 // If main thread uses lock-free containers
205 // the main thread should be attached to libcds infrastructure
206 cds::threading::Manager::attachThread();
208 // Now you can use RCU-based containers in the main thread
216 Each thread that deals with RCU-based container should be initialized first:
218 #include <cds/urcu/general_buffered.h>
219 int myThreadEntryPoint(void *)
221 // Attach the thread to libcds infrastructure
222 cds::threading::Manager::attachThread();
224 // Now you can use RCU-based containers in the thread
227 // Detach thread when terminating
228 cds::threading::Manager::detachThread();
234 # if CDS_OS_INTERFACE == CDS_OSI_UNIX || defined(CDS_DOXYGEN_INVOKED)
235 # define CDS_URCU_SIGNAL_HANDLING_ENABLED 1
238 /// General-purpose URCU type
239 struct general_purpose_rcu {
241 static uint32_t const c_nControlBit = 0x80000000;
242 static uint32_t const c_nNestMask = c_nControlBit - 1;
246 # ifdef CDS_URCU_SIGNAL_HANDLING_ENABLED
247 /// Signal-handling URCU type
248 struct signal_handling_rcu {
250 static uint32_t const c_nControlBit = 0x80000000;
251 static uint32_t const c_nNestMask = c_nControlBit - 1;
256 /// Tag for general_instant URCU
257 struct general_instant_tag: public general_purpose_rcu {
258 typedef general_purpose_rcu rcu_class ; ///< The URCU type
261 /// Tag for general_buffered URCU
262 struct general_buffered_tag: public general_purpose_rcu
264 typedef general_purpose_rcu rcu_class ; ///< The URCU type
267 /// Tag for general_threaded URCU
268 struct general_threaded_tag: public general_purpose_rcu {
269 typedef general_purpose_rcu rcu_class ; ///< The URCU type
272 # ifdef CDS_URCU_SIGNAL_HANDLING_ENABLED
273 /// Tag for signal_buffered URCU
274 struct signal_buffered_tag: public signal_handling_rcu {
275 typedef signal_handling_rcu rcu_class ; ///< The URCU type
278 /// Tag for signal_threaded URCU
279 struct signal_threaded_tag: public signal_handling_rcu {
280 typedef signal_handling_rcu rcu_class ; ///< The URCU type
284 ///@anchor cds_urcu_retired_ptr Retired pointer, i.e. pointer that ready for reclamation
285 typedef cds::gc::details::retired_ptr retired_ptr;
286 using cds::gc::make_retired_ptr;
288 /// Pointer to function to free (destruct and deallocate) retired pointer of specific type
289 typedef cds::gc::details::free_retired_ptr_func free_retired_ptr_func;
292 /// Implementation-specific URCU details
294 /// forward declarations
295 template <typename RCUtag>
298 template <typename RCUtag>
301 template <typename RCUtag >
305 class singleton_vtbl {
307 virtual ~singleton_vtbl()
310 virtual void retire_ptr( retired_ptr& p ) = 0;
316 template <typename MarkedPtr> using atomic_marked_ptr = atomics::atomic<MarkedPtr>;
321 template <typename ThreadData>
322 struct thread_list_record {
323 ThreadData * m_pNext; ///< Next item in thread list
324 atomics::atomic<OS::ThreadId> m_idOwner; ///< Owner thread id; 0 - the record is free (not owned)
328 , m_idOwner( cds::OS::c_NullThreadId )
331 ~thread_list_record()
337 template <typename RCUtag, class Alloc = CDS_DEFAULT_ALLOCATOR >
340 typedef thread_data<RCUtag> thread_record;
341 typedef cds::details::Allocator< thread_record, Alloc > allocator_type;
344 atomics::atomic<thread_record *> m_pHead;
356 thread_record * alloc()
358 thread_record * pRec;
359 cds::OS::ThreadId const nullThreadId = cds::OS::c_NullThreadId;
360 cds::OS::ThreadId const curThreadId = cds::OS::get_current_thread_id();
362 // First, try to reuse a retired (non-active) HP record
363 for ( pRec = m_pHead.load( atomics::memory_order_acquire ); pRec; pRec = pRec->m_list.m_pNext ) {
364 cds::OS::ThreadId thId = nullThreadId;
365 if ( !pRec->m_list.m_idOwner.compare_exchange_strong( thId, curThreadId, atomics::memory_order_seq_cst, atomics::memory_order_relaxed ) )
370 // No records available for reuse
371 // Allocate and push a new record
372 pRec = allocator_type().New();
373 pRec->m_list.m_idOwner.store( curThreadId, atomics::memory_order_relaxed );
375 thread_record * pOldHead = m_pHead.load( atomics::memory_order_acquire );
377 pRec->m_list.m_pNext = pOldHead;
378 } while ( !m_pHead.compare_exchange_weak( pOldHead, pRec, atomics::memory_order_release, atomics::memory_order_relaxed ));
383 void retire( thread_record * pRec )
385 assert( pRec != nullptr );
386 pRec->m_list.m_idOwner.store( cds::OS::c_NullThreadId, atomics::memory_order_release );
391 thread_record * pNext = nullptr;
392 cds::OS::ThreadId const nullThreadId = cds::OS::c_NullThreadId;
394 for ( thread_record * pRec = m_pHead.load(atomics::memory_order_acquire); pRec; pRec = pNext ) {
395 pNext = pRec->m_list.m_pNext;
396 if ( pRec->m_list.m_idOwner.load(atomics::memory_order_relaxed) != nullThreadId ) {
402 thread_record * head( atomics::memory_order mo ) const
404 return m_pHead.load( mo );
411 CDS_DEBUG_ONLY( cds::OS::ThreadId const nullThreadId = cds::OS::c_NullThreadId; )
412 CDS_DEBUG_ONLY( cds::OS::ThreadId const mainThreadId = cds::OS::get_current_thread_id() ;)
414 thread_record * p = m_pHead.exchange( nullptr, atomics::memory_order_acquire );
416 thread_record * pNext = p->m_list.m_pNext;
418 assert( p->m_list.m_idOwner.load( atomics::memory_order_relaxed ) == nullThreadId
419 || p->m_list.m_idOwner.load( atomics::memory_order_relaxed ) == mainThreadId
420 || !cds::OS::is_thread_alive( p->m_list.m_idOwner.load( atomics::memory_order_relaxed ) )
431 template <class ThreadGC>
434 typedef ThreadGC thread_gc;
435 typedef typename thread_gc::rcu_tag rcu_tag;
440 thread_gc::access_lock();
445 thread_gc::access_unlock();
449 } // namespace details
454 template <typename RCUimpl> class gc;
457 /// Epoch-based retired ptr
459 Retired pointer with additional epoch field that prevents early reclamation.
460 This type of retired pointer is used in buffered RCU implementations.
462 struct epoch_retired_ptr: public retired_ptr
464 uint64_t m_nEpoch; ///< The epoch when the object has been retired
471 /// Constructor creates a copy of \p rp retired pointer and saves \p nEpoch reclamation epoch.
472 epoch_retired_ptr( retired_ptr const& rp, uint64_t nEpoch )
481 #endif // #ifndef CDSLIB_URCU_DETAILS_BASE_H