openinx commented on a change in pull request #242: HBASE-22422 Retain an
ByteBuff with refCnt=0 when getBlock from LRUCache
URL: https://github.com/apache/hbase/pull/242#discussion_r287903698
##########
File path:
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/BlockCacheUtil.java
##########
@@ -230,6 +230,9 @@ public static boolean
shouldReplaceExistingCacheBlock(BlockCache blockCache,
BlockCacheKey cacheKey, Cacheable newBlock) {
// NOTICE: The getBlock has retained the existingBlock inside.
Cacheable existingBlock = blockCache.getBlock(cacheKey, false, false,
false);
+ if (existingBlock == null) {
Review comment:
Yes, see the LruBlockCache#cacheBlock:
```
LruCachedBlock cb = map.get(cacheKey);
if (cb != null && !BlockCacheUtil.shouldReplaceExistingCacheBlock(this,
cacheKey, buf)) {
return;
}
```
The existence pre-check and accessing block in
shouldReplaceExistingCacheBlock is not atomic op. so if any eviction happen
between them, the NPE will happen. I added a UT
testMultiThreadGetAndEvictBlock to address this.
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