SEGFAULT when using shared_ptr

Question

I am trying to implement a Lazy Concurrent List-Set Set in C ++ using shared_ptr. My reasoning is that it unreachable nodeswill be automatically released last shared_ptr. In my understanding, the operation of increasing and decreasing by a shared_ptr reference countis atomic. This means that only last shared_ptr with a link to a node should call delete / for free for that node. I started the program for several threads , but my program crashes with an error double free calledor just segmentation error (SIGSEGV) . I do not understand how this is possible. Below is my implementation code, with the method names representing their intended operation.

#include<thread>
#include<iostream>
#include<mutex>
#include<climits>

using namespace std;

class Thread
{
   public:
      std::thread t;  
};
int n=50,ki=100,kd=100,kc=100;`/*no of threads, no of inserts,deletes & searches*/`


class Node
{
public:
      int key;
      shared_ptr<Node> next;
      bool marked;
      std::mutex nodeLock;

      Node() {
         key=0;
         next = nullptr;
         marked = false;
      }

      Node(int k) {
         key = k;
         next = nullptr;
         marked = false;
      }

      void lock() {
         nodeLock.lock();
      }

      void unlock() {
         nodeLock.unlock();
      }

      ~Node()
      {
      }
};

class List {
   shared_ptr<Node> head;
   shared_ptr<Node> tail;

public:

   bool validate(shared_ptr<Node> pred, shared_ptr<Node> curr) {
      return !(pred->marked) && !(curr->marked) && ((pred->next) == curr);
   }

   List() {
      head=make_shared<Node>(INT_MIN);
      tail=make_shared<Node>(INT_MAX);
      head->next=tail;
   }

   bool add(int key)
   {
      while(true)
      {
         /*shared_ptr<Node> pred = head;
         shared_ptr<Node> curr = pred->next;*/
        auto pred = head;
        auto curr = pred->next;

         while (key>(curr->key))
         {
            pred = curr;
            curr = curr->next;
         }

         pred->lock();
         curr->lock();

         if (validate(pred,curr))
         {
            if (curr->key == key)
            {
               curr->unlock();
               pred->unlock();
               return false;
            }
            else
            {
                shared_ptr<Node> newNode(new Node(key));
               //auto newNode = make_shared<Node>(key);
                //shared_ptr<Node> newNode = make_shared<Node>(key);
                newNode->next = curr;
                pred->next = newNode;
                curr->unlock();
                pred->unlock();
                return true;
            }
         }
         curr->unlock();
         pred->unlock();
      }
   }

   bool remove(int key)
   {
      while(true)
      {
         /*shared_ptr<Node> pred = head;
         shared_ptr<Node> curr = pred->next;*/

        auto pred = head;
        auto curr = pred->next;

         while (key>(curr->key))
         {
            pred = curr;
            curr = curr->next;
         }

         pred->lock();
         curr->lock();

         if (validate(pred,curr))
         {
            if (curr->key != key)
            {
               curr->unlock();
               pred->unlock();
               return false;
            }
            else
            {
               curr->marked = true;
               pred->next = curr->next;
               curr->unlock();
               pred->unlock();
               return true;
            }
         }
         curr->unlock();
         pred->unlock();
      }
   }

   bool contains(int key) {
      //shared_ptr<Node> curr = head->next;
    auto curr = head->next;

      while (key>(curr->key)) {
         curr = curr->next;
      }
      return curr->key == key && !curr->marked;
   }
}list;

void test(int curr)
{
   bool test;
    int time;

    int val, choice;
    int total,k=0;
    total=ki+kd+kc;

    int i=0,d=0,c=0;

    while(k<total)
    {
        choice = (rand()%3)+1;

        if(choice==1)
        {
            if(i<ki)
            {
                val = (rand()%99)+1;
                test = list.add(val);
                i++;
                k++;
            }
        }
        else if(choice==2)
        {
            if(d<kd)
            {
                val = (rand()%99)+1;
                test = list.remove(val);
                d++;
                k++;
            }
        }
        else if(choice==3)
        {
            if(c<kc)
            {
                val = (rand()%99)+1;
                test = list.contains(val);
                c++;
                k++;
            }
        }
    }
}

int main()
{
   int i;

   vector<Thread>thr(n);

   for(i=0;i<n;i++)
   {
      thr[i].t = thread(test,i+1);
   }
   for(i=0;i<n;i++)
   {
      thr[i].t.join();
   }
   return 0;
}

, . , SEGFAULTS

pure virtual method called
terminate called without an active exception
Aborted (core dumped)

, ? ?
EDIT: test function, list methods. , . Crude programming, SEGFAULT.

Answer 1

, shared_ptr s.

, ( shared_ptr, , ) shared_ptr, , shared_ptr .

, shared_ptr , , shared_ptr, - -const, next.

( ) - libstd++ shared_ptr:

shared_ptr(const shared_ptr& rhs)
 : m_ptr(rhs.m_ptr),
   m_refcount(rhs.m_refcount) 
{ }

m_ptr , m_refcount - , , m_ptr.

, : , , . auto curr = head->next List::contains. , curr.m_ptr, , .

head. ref-count head->next - 1 ( , ref-count head->next 1), , node, .

. curr, m_ptr , 2 , node. key > curr->key 1 .

std:: atomic_load std:: atomic_store

std::atomic_load std::atomic_store copy-constructor/copy-assign-operator shared_ptr, . shared_ptr, , std::atomic_load/std::atomic_store . , m_ptr, , shared_ptr.

- List :

bool validate(Node const& pred, Node const& curr) {
   return !(pred.marked) && !(curr.marked) && 
          (std::atomic_load(&pred.next).get() == &curr);
}

bool add(int key) {
    while (true) {
        auto pred = std::atomic_load(&head);
        auto curr = std::atomic_load(&pred->next);

        while (key > (curr->key)) {
            pred = std::move(curr);
            curr = std::atomic_load(&pred->next);
        }

        std::scoped_lock lock{pred->nodeLock, curr->nodeLock};
        if (validate(*pred, *curr)) {
            if (curr->key == key) {
                return false;
            } else {
                auto new_node = std::make_shared<Node>(key);

                new_node->next = std::move(curr);
                std::atomic_store(&pred->next, std::move(new_node));
                return true;
            }
        }
    }
}

bool remove(int key) {
    while (true) {
        auto pred = std::atomic_load(&head);
        auto curr = std::atomic_load(&pred->next);

        while (key > (curr->key)) {
            pred = std::move(curr);
            curr = std::atomic_load(&pred->next);
        }

        std::scoped_lock lock{pred->nodeLock, curr->nodeLock};
        if (validate(*pred, *curr)) {
            if (curr->key != key) {
                return false;
            } else {
                curr->marked = true;
                std::atomic_store(&pred->next, std::atomic_load(&curr->next));
                return true;
            }
        }
    }
}

bool contains(int key) {
    auto curr = std::atomic_load(&head->next);

    while (key > (curr->key)) {
        curr = std::atomic_load(&curr->next);
    }
    return curr->key == key && !curr->marked;
}

, Node::marked a std::atomic_bool.