Optimize circuit breaking state transformation using CAS in DegradeRule (#538)

close → open: use `compareAndSet` to update for `cut` attribute (changed to an `AtomicBoolean`)

sentinel-core/src/main/java/com/alibaba/csp/sentinel/slots/block/degrade/DegradeRule.java

@@ -18,6 +18,7 @@ package com.alibaba.csp.sentinel.slots.block.degrade;
 import java.util.concurrent.Executors;
 import java.util.concurrent.ScheduledExecutorService;
 import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicLong;
 
 import com.alibaba.csp.sentinel.concurrent.NamedThreadFactory;
@@ -80,7 +81,7 @@ public class DegradeRule extends AbstractRule {
      */
     private int grade = RuleConstant.DEGRADE_GRADE_RT;
 
-    private volatile boolean cut = false;
+    private final AtomicBoolean cut = new AtomicBoolean(false);
 
     public int getGrade() {
         return grade;
@@ -93,8 +94,6 @@ public class DegradeRule extends AbstractRule {
 
     private AtomicLong passCount = new AtomicLong(0);
 
-    private final Object lock = new Object();
-
     public double getCount() {
         return count;
     }
@@ -104,12 +103,12 @@ public class DegradeRule extends AbstractRule {
         return this;
     }
 
-    public boolean isCut() {
-        return cut;
+    private boolean isCut() {
+        return cut.get();
     }
 
     private void setCut(boolean cut) {
-        this.cut = cut;
+        this.cut.set(cut);
     }
 
     public AtomicLong getPassCount() {
@@ -162,7 +161,7 @@ public class DegradeRule extends AbstractRule {
 
     @Override
     public boolean passCheck(Context context, DefaultNode node, int acquireCount, Object... args) {
-        if (cut) {
+        if (cut.get()) {
             return false;
         }
 
@@ -206,16 +205,12 @@ public class DegradeRule extends AbstractRule {
             }
         }
 
-        synchronized (lock) {
-            if (!cut) {
-                // Automatically degrade.
-                cut = true;
-                ResetTask resetTask = new ResetTask(this);
-                pool.schedule(resetTask, timeWindow, TimeUnit.SECONDS);
-            }
-
-            return false;
+        if (cut.compareAndSet(false, true)) {
+            ResetTask resetTask = new ResetTask(this);
+            pool.schedule(resetTask, timeWindow, TimeUnit.SECONDS);
         }
+
+        return false;
     }
 
     @Override
@@ -240,7 +235,7 @@ public class DegradeRule extends AbstractRule {
         @Override
         public void run() {
             rule.getPassCount().set(0);
-            rule.setCut(false);
+            rule.cut.set(false);
         }
     }
 }