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Make DTZEntry init() thread safe

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This commit is contained in:
Marco Costalba 2016-05-26 10:38:02 +02:00
parent 5ebd56ebd8
commit ff3165286c
1 changed files with 19 additions and 15 deletions
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28
src/syzygy/tbprobe.cpp
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@ -128,7 +128,7 @@ struct PairsData {
uint8_t groupLen[TBPIECES]; // Number of pieces in a given group: KRKN -> (3) + (1) uint8_t groupLen[TBPIECES]; // Number of pieces in a given group: KRKN -> (3) + (1)
}; };
// Helper struct to avoid to manually define WDLEntry copy c'tor as we should // Helper struct to avoid to manually define entry copy c'tor as we should
// because default one is not compatible with std::atomic_bool. // because default one is not compatible with std::atomic_bool.
struct Atomic { struct Atomic {
Atomic() = default; Atomic() = default;
@ -161,7 +161,7 @@ struct WDLEntry : public Atomic {
}; };
}; };
struct DTZEntry { struct DTZEntry : public Atomic {
DTZEntry(const WDLEntry& wdl); DTZEntry(const WDLEntry& wdl);
~DTZEntry(); ~DTZEntry();
@ -442,6 +442,8 @@ DTZEntry::DTZEntry(const WDLEntry& wdl) {
memset(this, 0, sizeof(DTZEntry)); memset(this, 0, sizeof(DTZEntry));
ready = false;
key = wdl.key; key = wdl.key;
key2 = wdl.key2; key2 = wdl.key2;
pieceCount = wdl.pieceCount; pieceCount = wdl.pieceCount;
@ -1108,6 +1110,16 @@ bool init(Entry& e, const Position& pos)
const bool IsWDL = std::is_same<Entry, WDLEntry>::value; const bool IsWDL = std::is_same<Entry, WDLEntry>::value;
const uint8_t* MAGIC = IsWDL ? WDL_MAGIC : DTZ_MAGIC; const uint8_t* MAGIC = IsWDL ? WDL_MAGIC : DTZ_MAGIC;
// Avoid a thread reads 'ready' == true while another is still in do_init(),
// this could happen due to compiler reordering.
if (e.ready.load(std::memory_order_acquire))
return true;
std::unique_lock<Mutex> lk(TB_mutex);
if (e.ready.load(std::memory_order_relaxed)) // Recheck under lock
return true;
std::string fname, w, b; std::string fname, w, b;
// Position pieces in decreasing order for each color, like ("KPP","KR") // Position pieces in decreasing order for each color, like ("KPP","KR")
@ -1124,6 +1136,7 @@ bool init(Entry& e, const Position& pos)
return false; return false;
e.hasPawns ? do_init(e, e.pawn, data) : do_init(e, e.piece, data); e.hasPawns ? do_init(e, e.pawn, data) : do_init(e, e.piece, data);
e.ready.store(true, std::memory_order_release);
return true; return true;
} }
@ -1140,20 +1153,11 @@ WDLScore probe_wdl_table(Position& pos, ProbeState* result) {
return WDLDraw; return WDLDraw;
} }
// Init table at first access attempt. Special care should be taken to avoid // Init table at first access attempt, init() is thread safe
// one thread reads ready == 1 while the other is still in init(), this could
// happen due to compiler reordering.
if (!entry->ready.load(std::memory_order_acquire)) {
std::unique_lock<Mutex> lk(TB_mutex);
if (!entry->ready.load(std::memory_order_relaxed)) {
if (!init(*entry, pos)) { if (!init(*entry, pos)) {
// Was ptr2->key = 0ULL; Just leave !ptr->ready condition
*result = FAIL; *result = FAIL;
return WDLDraw; return WDLDraw;
} }
entry->ready.store(1, std::memory_order_release);
}
}
return (WDLScore)probe_table(pos, entry); return (WDLScore)probe_table(pos, entry);
} }