implemented hazard pointer management

memory/hp.hpp

@@ -0,0 +1,112 @@
+#ifndef MEMGRAPH_MEMORY_HP_HPP
+#define MEMGRAPH_MEMORY_HP_HPP
+
+#include <atomic>
+#include <cassert>
+#include <unistd.h>
+
+namespace memory
+{
+
+constexpr const size_t HP_SIZE = 128;
+
+class HP
+{
+public:
+    // this object can't be copied or moved
+    HP(HP&) = delete;
+    HP(HP&&) = delete;
+
+    // grabs a singleton instance
+    static HP& get()
+    {
+        static HP hp;
+        return hp;
+    }
+
+    class reference
+    {
+        friend class HP;
+    public:
+        reference(reference&) = delete;
+
+        // this type shouldn't be copyable to avoid calling its destructor
+        // multiple times, but should be movable
+        reference(reference&& other)
+        {
+            this->idx = other.idx;
+
+            // set the index to a negative number to indicate that this
+            // index has been moved and that you should not free its
+            // hazard pointer
+            other.idx = -1;
+        }
+        // hazard pointer is cleared once reference goes out of scope
+        ~reference()
+        {
+            // check if this reference was moved during its lifetime
+            if(idx < 0)
+                return;
+
+            auto& hp = HP::get();
+            hp.clear(*this);
+        }
+
+    private:
+        reference(int64_t idx) : idx(idx) {}
+        int64_t idx;
+    };
+
+    friend class reference;
+
+    template <class T>
+    reference insert(T* ptr)
+    {
+        auto p = reinterpret_cast<uintptr_t>(ptr);
+
+        while(true)
+        {
+            // try to find a free spot in the hazard pointer list
+            for(size_t i = 0; i < HP_SIZE; ++i)
+            {
+                auto hazard = ptr_list[i].load();
+
+                // if this spot isn't free, continue searching
+                if(hazard != 0)
+                    continue;
+
+                // try to take this spot, if we fail, then another thread has
+                // just taken it. continue searching for a new one
+                if(!ptr_list[i].compare_exchange_strong(hazard, p))
+                    continue;
+
+                // found a free spot! return a reference to this spot so it
+                // can be cleared later
+                return reference(i);
+            }
+
+            // we didn't find any free spots, sleep for a while and try again
+            // from the beginning, some other thread might have freed a spot
+            // while we were traversing the lsit.
+            usleep(250);
+        }
+    }
+
+private:
+    HP()
+    {
+        for(size_t i = 0; i < HP_SIZE; ++i)
+            ptr_list[i].store(0);
+    }
+
+    void clear(reference& ref)
+    {
+        ptr_list[ref.idx].store(0);
+    }
+
+    std::atomic<uintptr_t> ptr_list[HP_SIZE];
+};
+
+}
+
+#endif