62 }
63
64 uintx G1FreeIdSet::make_head(uint index, uintx old_head) const {
65 // Include incremented old update counter to avoid ABA problem.
66 return index | ((old_head & ~_head_index_mask) + 1 + _head_index_mask);
67 }
68
69 const uint Claimed = UINT_MAX;
70
71 uint G1FreeIdSet::claim_par_id() {
72 _sem.wait();
73 // Semaphore gate permits passage by no more than the number of
74 // available ids, so there must be one that we can claim. But there
75 // may be multiple threads trying to claim ids at the same time.
76 uintx old_head = Atomic::load(&_head);
77 uint index;
78 while (true) {
79 index = head_index(old_head);
80 assert(index < _size, "invariant");
81 uintx new_head = make_head(_next[index], old_head);
82 new_head = Atomic::cmpxchg(new_head, &_head, old_head);
83 if (new_head == old_head) break;
84 old_head = new_head;
85 }
86 DEBUG_ONLY(_next[index] = Claimed;)
87 return _start + index;
88 }
89
90 void G1FreeIdSet::release_par_id(uint id) {
91 uint index = id - _start;
92 assert(index < _size, "invalid id %u", id);
93 assert(_next[index] == Claimed, "precondition");
94 uintx old_head = Atomic::load(&_head);
95 while (true) {
96 _next[index] = head_index(old_head);
97 uintx new_head = make_head(index, old_head);
98 new_head = Atomic::cmpxchg(new_head, &_head, old_head);
99 if (new_head == old_head) break;
100 old_head = new_head;
101 }
102 // Now that id has been released, permit another thread through the gate.
103 _sem.signal();
104 }
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62 }
63
64 uintx G1FreeIdSet::make_head(uint index, uintx old_head) const {
65 // Include incremented old update counter to avoid ABA problem.
66 return index | ((old_head & ~_head_index_mask) + 1 + _head_index_mask);
67 }
68
69 const uint Claimed = UINT_MAX;
70
71 uint G1FreeIdSet::claim_par_id() {
72 _sem.wait();
73 // Semaphore gate permits passage by no more than the number of
74 // available ids, so there must be one that we can claim. But there
75 // may be multiple threads trying to claim ids at the same time.
76 uintx old_head = Atomic::load(&_head);
77 uint index;
78 while (true) {
79 index = head_index(old_head);
80 assert(index < _size, "invariant");
81 uintx new_head = make_head(_next[index], old_head);
82 new_head = Atomic::cmpxchg(&_head, old_head, new_head);
83 if (new_head == old_head) break;
84 old_head = new_head;
85 }
86 DEBUG_ONLY(_next[index] = Claimed;)
87 return _start + index;
88 }
89
90 void G1FreeIdSet::release_par_id(uint id) {
91 uint index = id - _start;
92 assert(index < _size, "invalid id %u", id);
93 assert(_next[index] == Claimed, "precondition");
94 uintx old_head = Atomic::load(&_head);
95 while (true) {
96 _next[index] = head_index(old_head);
97 uintx new_head = make_head(index, old_head);
98 new_head = Atomic::cmpxchg(&_head, old_head, new_head);
99 if (new_head == old_head) break;
100 old_head = new_head;
101 }
102 // Now that id has been released, permit another thread through the gate.
103 _sem.signal();
104 }
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