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handle spurious wake ups in BarrierSemaphore #82

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handle spurious wake ups in BarrierSemaphore #82

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23 changes: 19 additions & 4 deletions src/parallel_solve.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -36,7 +36,8 @@ namespace Clasp { namespace mt {
// A combination of a barrier and a semaphore
class BarrierSemaphore {
public:
explicit BarrierSemaphore(int counter = 0, int maxParties = 1) : counter_(counter), active_(maxParties) {}
explicit BarrierSemaphore(int counter = 0, int maxParties = 1)
: counter_(counter), active_(maxParties), wait_{0} , notify_{0} {}
// Initializes this object
// PRE: no thread is blocked on the semaphore
// (i.e. internal counter is >= 0)
Expand Down Expand Up @@ -68,7 +69,12 @@ class BarrierSemaphore {
assert(active_ > 0);
int res = active_--;
if (reset) { unsafe_reset(0); }
else if (unsafe_active()) { counter_ = -active_; lock.unlock(); semCond_.notify_one(); }
else if (unsafe_active()) {
counter_ = -active_;
if (notify_ < wait_) { ++notify_; }
lock.unlock();
semCond_.notify_one();
}
return res;
}
// Waits until all parties have arrived, i.e. called wait.
Expand Down Expand Up @@ -114,6 +120,7 @@ class BarrierSemaphore {
void up(unique_lock<mutex>& m, bool transferLock = true) {
assert(m.owns_lock());
if (++counter_ < 1) {
if (notify_ < wait_) { ++notify_; }
if (transferLock) { m.unlock(); }
semCond_.notify_one();
}
Expand All @@ -127,21 +134,29 @@ class BarrierSemaphore {
void unsafe_reset(uint32 semCount) {
int prev = counter_;
counter_ = semCount;
if (prev < 0) { semCond_.notify_all(); }
if (prev < 0) {
notify_ = wait_;
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Should the mutex be unlocked before notification?

semCond_.notify_all();
}
}
// Returns true for the leader, else false
bool unsafe_wait(unique_lock<mutex>& lock) {
assert(counter_ < 0);
// don't put the last thread to sleep!
if (!unsafe_active()) {
semCond_.wait(lock);
++wait_;
while (notify_ == 0) { semCond_.wait(lock); }
--notify_;
--wait_;
}
return unsafe_active();
}
cv semCond_; // waiting threads
mutex semMutex_; // mutex for updating counter
int counter_; // semaphore's counter
int active_; // number of active threads
int wait_; // number of currently waiting threads
int notify_; // number of unprocessed notifications
};
/////////////////////////////////////////////////////////////////////////////////////////
// ParallelSolve::Impl
Expand Down