1 /*
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  11  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  12  * version 2 for more details (a copy is included in the LICENSE file that
  13  * accompanied this code).
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  24 
  25 #ifndef SHARE_GC_SHARED_OOPSTORAGEPARSTATE_HPP
  26 #define SHARE_GC_SHARED_OOPSTORAGEPARSTATE_HPP
  27 
  28 #include "gc/shared/oopStorage.hpp"
  29 #include "utilities/macros.hpp"
  30 
  31 //////////////////////////////////////////////////////////////////////////////
  32 // Support for parallel and optionally concurrent state iteration.
  33 //
  34 // Parallel iteration is for the exclusive use of the GC.  Other iteration
  35 // clients must use serial iteration.
  36 //
  37 // Concurrent Iteration
  38 //
  39 // Iteration involves the _active_array (an ActiveArray), which contains all
  40 // of the blocks owned by a storage object.
  41 //
  42 // At most one concurrent ParState can exist at a time for a given storage
  43 // object.
  44 //
  45 // A concurrent ParState sets the associated storage's
  46 // _concurrent_iteration_active flag true when the state is constructed, and
  47 // sets it false when the state is destroyed.  These assignments are made with
  48 // _active_mutex locked.  Meanwhile, empty block deletion is not done while
  49 // _concurrent_iteration_active is true.  The flag check and the dependent
  50 // removal of a block from the _active_array is performed with _active_mutex
  51 // locked.  This prevents concurrent iteration and empty block deletion from
  52 // interfering with with each other.
  53 //
  54 // Both allocate() and delete_empty_blocks_concurrent() lock the
  55 // _allocation_mutex while performing their respective list and array
  56 // manipulations, preventing them from interfering with each other.
  57 //
  58 // When allocate() creates a new block, it is added to the end of the
  59 // _active_array.  Then _active_array's _block_count is incremented to account
  60 // for the new block.  When concurrent iteration is started (by a parallel
  61 // worker thread calling the state's iterate() function), the current
  62 // _active_array and its _block_count are captured for use by the iteration,
  63 // with iteration processing all blocks in that array up to that block count.
  64 //
  65 // As a result, the sequence over which concurrent iteration operates is
  66 // stable.  However, once the iteration is started, later allocations may add
  67 // blocks to the end of the array that won't be examined by the iteration.
  68 // An allocation may even require expansion of the array, so the iteration is
  69 // no longer processing the current array, but rather the previous one.
  70 // And while the sequence is stable, concurrent allocate() and release()
  71 // operations may change the set of allocated entries in a block at any time
  72 // during the iteration.
  73 //
  74 // As a result, a concurrent iteration handler must accept that some
  75 // allocations and releases that occur after the iteration started will not be
  76 // seen by the iteration.  Further, some may overlap examination by the
  77 // iteration.  To help with this, allocate() and release() have an invariant
  78 // that an entry's value must be NULL when it is not in use.
  79 //
  80 // An in-progress delete_empty_blocks_concurrent() operation can contend with
  81 // the start of a concurrent iteration over the _active_mutex.  Since both are
  82 // under GC control, that potential contention can be eliminated by never
  83 // scheduling both operations to run at the same time.
  84 //
  85 // ParState<concurrent, is_const>
  86 //   concurrent must be true if iteration is concurrent with the
  87 //   mutator, false if iteration is at a safepoint.
  88 //
  89 //   is_const must be true if the iteration is over a constant storage
  90 //   object, false if the iteration may modify the storage object.
  91 //
  92 // ParState([const] OopStorage* storage)
  93 //   Construct an object for managing an iteration over storage.  For a
  94 //   concurrent ParState, empty block deletion for the associated storage
  95 //   is inhibited for the life of the ParState.  There can be no more
  96 //   than one live concurrent ParState at a time for a given storage object.
  97 //
  98 // template<typename F> void iterate(F f)
  99 //   Repeatedly claims a block from the associated storage that has
 100 //   not been processed by this iteration (possibly by other threads),
 101 //   and applies f to each entry in the claimed block. Assume p is of
 102 //   type const oop* or oop*, according to is_const. Then f(p) must be
 103 //   a valid expression whose value is ignored.  Concurrent uses must
 104 //   be prepared for an entry's value to change at any time, due to
 105 //   mutator activity.
 106 //
 107 // template<typename Closure> void oops_do(Closure* cl)
 108 //   Wrapper around iterate, providing an adaptation layer allowing
 109 //   the use of OopClosures and similar objects for iteration.  Assume
 110 //   p is of type const oop* or oop*, according to is_const.  Then
 111 //   cl->do_oop(p) must be a valid expression whose value is ignored.
 112 //   Concurrent uses must be prepared for the entry's value to change
 113 //   at any time, due to mutator activity.
 114 //
 115 // Optional operations, provided only if !concurrent && !is_const.
 116 // These are not provided when is_const, because the storage object
 117 // may be modified by the iteration infrastructure, even if the
 118 // provided closure doesn't modify the storage object.  These are not
 119 // provided when concurrent because any pre-filtering behavior by the
 120 // iteration infrastructure is inappropriate for concurrent iteration;
 121 // modifications of the storage by the mutator could result in the
 122 // pre-filtering being applied (successfully or not) to objects that
 123 // are unrelated to what the closure finds in the entry.
 124 //
 125 // template<typename Closure> void weak_oops_do(Closure* cl)
 126 // template<typename IsAliveClosure, typename Closure>
 127 // void weak_oops_do(IsAliveClosure* is_alive, Closure* cl)
 128 //   Wrappers around iterate, providing an adaptation layer allowing
 129 //   the use of is-alive closures and OopClosures for iteration.
 130 //   Assume p is of type oop*.  Then
 131 //
 132 //   - cl->do_oop(p) must be a valid expression whose value is ignored.
 133 //
 134 //   - is_alive->do_object_b(*p) must be a valid expression whose value
 135 //   is convertible to bool.
 136 //
 137 //   If *p == NULL then neither is_alive nor cl will be invoked for p.
 138 //   If is_alive->do_object_b(*p) is false, then cl will not be
 139 //   invoked on p.
 140 
 141 class OopStorage::BasicParState {
 142   const OopStorage* _storage;
 143   ActiveArray* _active_array;
 144   size_t _block_count;
 145   volatile size_t _next_block;
 146   uint _estimated_thread_count;
 147   bool _concurrent;
 148 
 149   // Noncopyable.
 150   BasicParState(const BasicParState&);
 151   BasicParState& operator=(const BasicParState&);
 152 
 153   struct IterationData;
 154 
 155   void update_iteration_state(bool value);
 156   bool claim_next_segment(IterationData* data);
 157   bool finish_iteration(const IterationData* data) const;
 158 
 159   // Wrapper for iteration handler; ignore handler result and return true.
 160   template<typename F> class AlwaysTrueFn;
 161 
 162 public:
 163   BasicParState(const OopStorage* storage,
 164                 uint estimated_thread_count,
 165                 bool concurrent);
 166   ~BasicParState();
 167 
 168   template<bool is_const, typename F> void iterate(F f);
 169 
 170   static uint default_estimated_thread_count(bool concurrent);
 171 };
 172 
 173 template<bool concurrent, bool is_const>
 174 class OopStorage::ParState {
 175   BasicParState _basic_state;
 176 
 177   typedef typename Conditional<is_const,
 178                                const OopStorage*,
 179                                OopStorage*>::type StoragePtr;
 180 
 181 public:
 182   ParState(StoragePtr storage,
 183            uint estimated_thread_count = BasicParState::default_estimated_thread_count(concurrent)) :
 184     _basic_state(storage, estimated_thread_count, concurrent)
 185   {}
 186 
 187   template<typename F> void iterate(F f);
 188   template<typename Closure> void oops_do(Closure* cl);
 189 };
 190 
 191 template<>
 192 class OopStorage::ParState<false, false> {
 193   BasicParState _basic_state;
 194 
 195 public:
 196   ParState(OopStorage* storage,
 197            uint estimated_thread_count = BasicParState::default_estimated_thread_count(false)) :
 198     _basic_state(storage, estimated_thread_count, false)
 199   {}
 200 
 201   template<typename F> void iterate(F f);
 202   template<typename Closure> void oops_do(Closure* cl);
 203   template<typename Closure> void weak_oops_do(Closure* cl);
 204   template<typename IsAliveClosure, typename Closure>
 205   void weak_oops_do(IsAliveClosure* is_alive, Closure* cl);
 206 };
 207 
 208 #endif // SHARE_GC_SHARED_OOPSTORAGEPARSTATE_HPP