virajjasani commented on a change in pull request #3407:
URL: https://github.com/apache/hbase/pull/3407#discussion_r687501045



##########
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:
       > A cached buffer item detached from the cache still needs to have its 
#release called w/ refcount at zero so the backing memory gets readded to the 
pool.
   
   Yeah I think this makes sense. Let me get back to this in case I find some 
better and obvious way to improve perf and get some YCSB results.




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