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  1. JDK
  2. JDK-8259021

SharedSecrets should avoid double racy reads from non-volatile fields

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Details

    • Bug
    • Resolution: Fixed
    • P4
    • 17
    • None
    • client-libs
    • None
    • b04

    Description

      Some XxxAccess instances in jdk.internal.access.SharedSecrets are initialized lazily on 1st access with the following idiom:

          public static JavaUtilJarAccess javaUtilJarAccess() {
              if (javaUtilJarAccess == null) {
                  // Ensure JarFile is initialized; we know that this class
                  // provides the shared secret
                  ensureClassInitialized(JarFile.class);
              }
              return javaUtilJarAccess;
          }

      But this idiom is flawed in concurrent setting. It may happen that the two reads of javaUtilJarAccess non-volatile field get reordered and so the 1st read (in if condition) reads non-null while the 2nd read (in return) reads null. This can be fixed by introducing a local variable that avoids the 2nd read:

          public static JavaUtilJarAccess javaUtilJarAccess() {
              var access = javaUtilJarAccess;
              if (access == null) {
                  // Ensure JarFile is initialized; we know that this class
                  // provides the shared secret
                  ensureClassInitialized(JarFile.class);
                  access = javaUtilJarAccess;
              }
              return access;
          }

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              plevart Peter Levart
              plevart Peter Levart
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                Updated:
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