toddlipcon commented on a change in pull request #595: HDFS-14304: High lock
contention on hdfsHashMutex in libhdfs
URL: https://github.com/apache/hadoop/pull/595#discussion_r267053653
##########
File path:
hadoop-hdfs-project/hadoop-hdfs-native-client/src/main/native/libhdfs/jni_helper.c
##########
@@ -534,6 +545,21 @@ JNIEnv* getJNIEnv(void)
state->env = getGlobalJNIEnv();
mutexUnlock(&jvmMutex);
+
+ if (!jclassesInitialized) {
+ mutexLock(&jclassInitMutex);
+ if (!jclassesInitialized) {
Review comment:
this double-checked locking idiom isn't safe unless you insert a memory
barrier before the store to jclassesInitialized. (it has to be a store with
"release" semantics)
Otherwise, the compiler (and/or CPU) is free to decide to reorder the store
to 'jclassesInitialized = 1' up earlier within the critical section.
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