virajjasani commented on a change in pull request #3407:
URL: https://github.com/apache/hbase/pull/3407#discussion_r671717888



##########
File path: 
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/LruAdaptiveBlockCache.java
##########
@@ -646,14 +639,16 @@ private long updateSizeMetrics(LruCachedBlock cb, boolean 
evict) {
   @Override
   public Cacheable getBlock(BlockCacheKey cacheKey, boolean caching, boolean 
repeat,
     boolean updateCacheMetrics) {
-    LruCachedBlock cb = map.computeIfPresent(cacheKey, (key, val) -> {
-      // It will be referenced by RPC path, so increase here. NOTICE: Must do 
the retain inside
-      // this block. because if retain outside the map#computeIfPresent, the 
evictBlock may remove
-      // the block and release, then we're retaining a block with refCnt=0 
which is disallowed.
-      // see HBASE-22422.
-      val.getBuffer().retain();
-      return val;
-    });
+    LruCachedBlock cb = map.get(cacheKey);
+    if (cb != null) {
+      try {
+        cb.getBuffer().retain();

Review comment:
       > Does the CB get returned to the cache when done?
   
   You mean if CB gets returned to L1 cache (CHM) after it's buffer has served 
read request? Yes, that's the case (unless I misunderstood the question)




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