[GitHub] [hbase] ben-manes commented on a change in pull request #3215: HBASE-25698 Fixing IllegalReferenceCountException when using TinyLfuBlockCache

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ben-manes commented on a change in pull request #3215:
URL: https://github.com/apache/hbase/pull/3215#discussion_r648556289



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File path: hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/TinyLfuBlockCache.java
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@@ -158,7 +158,13 @@ public boolean containsBlock(BlockCacheKey cacheKey) {
   @Override
   public Cacheable getBlock(BlockCacheKey cacheKey,
       boolean caching, boolean repeat, boolean updateCacheMetrics) {
-    Cacheable value = cache.getIfPresent(cacheKey);
+    Cacheable value = cache.asMap().computeIfPresent(cacheKey, (blockCacheKey, cacheable) -> {
+      // 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.
+      cacheable.retain();
+      return cacheable;
+    });

Review comment:
       I would switch to `map.remove(cacheKey, cb)` so that a race doesn't discard a new mapping. If my naive reading is correct, this `map.remove(cacheKey)` would already occur before the `cb.release()`, so this may not be necessary. That could mean that a new block was computed, so the remove discards that mistakenly. You might not need the map removal here if you can rely on the release being performed after the map operation completed.
   
   https://github.com/apache/hbase/blob/947c03cf7249dec09162da445df7d36b8dbd4bfc/hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/TinyLfuBlockCache.java#L246-L252




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