我在下面的代码中调试网格锁时遇到问题（用于计算图形的生成林，不一定是最小生成林）。我在代码中一直小心，总是首先获得具有更高索引的节点的锁，但是当我运行它时我仍然遇到死锁。我想知道我是否知道我的锁定策略出了什么问题？我尝试使用Eclipse调试器进行调试，但是没有什么能够突出显示可能导致它的原因。
链接到完整代码：https://github.com/lstrait2/parallel_SF?files=1
public class SFRunnable implements Runnable
{
private int start_idx;
private int end_idx;
private Graph.Node[] u_ancestors;
private Graph.Node[] v_ancestors;
private ArrayList<Graph.Edge> edges;
private ConcurrentHashMap<Graph.Node,Graph.Node> newAncestors;
public SFRunnable(
int start,
int end,
ArrayList<Graph.Edge> edges,
Graph.Node[] u_ancestors,
Graph.Node[] v_ancestors,
ConcurrentHashMap<Graph.Node,Graph.Node> newAncestors
) {
this.start_idx = start;
this.end_idx = end;
this.edges = edges;
this.u_ancestors = u_ancestors;
this.v_ancestors = v_ancestors;
this.newAncestors = newAncestors;
}
@Override
public void run()
{
for(int i = this.start_idx; i < this.end_idx; i++)
{
Graph.Edge e = this.edges.get(i);
buildSF(i,this.u_ancestors,this.v_ancestors,this.newAncestors);
}
}
private Graph.Node ancestorOf(
final Graph.Node u,
ConcurrentHashMap<Graph.Node, Graph.Node> newAncestors
) {
Graph.Node newAncestor;
if(u.ancestor != u)
{
return u.ancestor;
}
else if( (newAncestor = newAncestors.get(u)) != null)
{
return newAncestor;
}
return u;
}
private void buildSF(
int i,
Graph.Node[] u_ancestors,
Graph.Node[] v_ancestors,
ConcurrentHashMap<Graph.Node, Graph.Node> newAncestors
) {
Graph.Edge e = this.edges.get(i);
if(e.u.index < e.v.index)
{
synchronized(e.v)
{
Graph.Node v_ancestor = e.v;
synchronized(e.u)
{
Graph.Node u_ancestor = e.u;
buildSF(e,i,u_ancestors,v_ancestors,newAncestors,u_ancestor,v_ancestor);
}
}
}
else
{
synchronized(e.u)
{
Graph.Node u_ancestor = e.u;
synchronized(e.v)
{
Graph.Node v_ancestor = e.v;
buildSF(e,i,u_ancestors,v_ancestors,newAncestors,u_ancestor,v_ancestor);
}
}
}
}
private void buildSF(
Graph.Edge e,
int i,
Graph.Node[] u_ancestors,
Graph.Node[] v_ancestors,
ConcurrentHashMap<Graph.Node,Graph.Node> newAncestors,
Graph.Node u_ancestor,
Graph.Node v_ancestor
) {
// locks acquired for u_ancestor and v_ancestor in previous call
Graph.Node nextAncestor_u = ancestorOf(u_ancestor,newAncestors);
Graph.Node nextAncestor_v = ancestorOf(v_ancestor,newAncestors);
if(nextAncestor_u == u_ancestor && nextAncestor_v == v_ancestor)
{
if(u_ancestor == v_ancestor)
return;
if(u_ancestor.index < v_ancestor.index)
{
// swap nodes
Graph.Node temp = u_ancestor;
u_ancestor = v_ancestor;
v_ancestor = temp;
}
u_ancestors[i] = u_ancestor;
v_ancestors[i] = v_ancestor;
newAncestors.put(u_ancestor,v_ancestor);
}
else
{
if(nextAncestor_u == u_ancestor)
{
synchronized(nextAncestor_v)
{
buildSF(e,i,u_ancestors,v_ancestors,newAncestors,u_ancestor,nextAncestor_v);
}
}
else if(nextAncestor_v == v_ancestor)
{
synchronized(nextAncestor_u)
{
buildSF(e,i,u_ancestors,v_ancestors,newAncestors,nextAncestor_u,v_ancestor);
}
}
else if(nextAncestor_u.index < nextAncestor_v.index)
{
synchronized(nextAncestor_v)
{
buildSF(e,i,u_ancestors,v_ancestors,newAncestors,u_ancestor,nextAncestor_v);
}
}
else
{
synchronized(nextAncestor_u)
{
buildSF(e,i,u_ancestors,v_ancestors,newAncestors,nextAncestor_u,v_ancestor);
}
}
}
}
这是图表代码：
import java.io.*;
import java.util.*;
public class Graph
{
static class Node
{
final int index;
Node ancestor;
Node(int index)
{
this.index = index;
ancestor = this;
}
}
static class Edge
{
final Node u, v;
boolean inSF = false;
Edge(Node u, Node v)
{
this.u = u;
this.v = v;
}
}
ArrayList<Edge> edges;
Node[] nodes;
public Graph()
{
edges = new ArrayList<>();
nodes = new Node[0];
}
static Graph readEdgeGraph(String file) throws IOException
{
BufferedReader reader = new BufferedReader(new FileReader(file));
Graph g = new Graph();
if(!"EdgeArray".equals(reader.readLine()))
throw new IOException("invalid edge graph format");
while(true)
{
String line;
try
{
line = reader.readLine();
}catch(EOFException e){ break;}
if(line == null)
break;
String[] words = line.split("[ \t]+");
if(words.length != 2)
throw new IOException("invalid edge graph format");
int u = Integer.parseInt(words[0]);
int v = Integer.parseInt(words[1]);
Node U = g.getVertex(u);
Node V = g.getVertex(v);
g.addEdge(U,V);
}
return g;
}
public Node getVertex(int n)
{
if(nodes.length < n)
nodes = Arrays.copyOf(nodes, n+1+ n/2);
if(nodes[n] == null)
nodes[n] = new Node(n);
return nodes[n];
}
public void addEdge(Node u, Node v)
{
Edge edge = new Edge(u,v);
edges.add(edge);
}
}
答案 0 :(得分：3)
在第一个buildSF方法中，e.u.index可能等于e.v.index。在这种情况下，两个线程可能会尝试以相反的顺序锁定具有相等索引的两个Graph.Node对象，这可能会导致死锁。
此外，在synchronized块中，您调用另一个buildSF方法，该方法在第三个Graph.Node上同步;第三个Graph.Node可能没有比线程已经锁定的两个Graph.Node对象更低的索引，因此也可能导致死锁。