RFR: 8266310: deadlock while loading the JNI code

[Peter Levart]

On Tue, 11 May 2021 13:10:30 GMT, Aleksei Voitylov <avoitylov at openjdk.org> wrote:

> Please review this PR which fixes the deadlock in ClassLoader between the two lock objects - a lock object associated with the class being loaded, and the ClassLoader.loadedLibraryNames hash map, locked during the native library load operation.
> 
> Problem being fixed:
> 
> The initial reproducer demonstrated a deadlock between the JarFile/ZipFile and the hash map. That deadlock exists even when the ZipFile/JarFile lock is removed because there's another lock object in the class loader, associated with the name of the class being loaded. Such objects are stored in ClassLoader.parallelLockMap. The deadlock occurs when JNI_OnLoad() loads exactly the same class, whose signature is being verified in another thread.
> 
> Proposed fix:
> 
> The proposed patch suggests to get rid of locking loadedLibraryNames hash map and synchronize on each entry name, as it's done with class names in see ClassLoader.getClassLoadingLock(name) method.
> 
> The patch introduces nativeLibraryLockMap which holds the lock objects for each library name, and the getNativeLibraryLock() private method is used to lazily initialize the corresponding lock object. nativeLibraryContext was changed to ThreadLocal, so that in any concurrent thread it would have a NativeLibrary object on top of the stack, that's being currently loaded/unloaded in that thread. nativeLibraryLockMap accumulates the names of all native libraries loaded - in line with class loading code, it is not explicitly cleared.
> 
> Testing:  jtreg and jck testing with no regressions. A new regression test was developed.

Hi Aleksei,
If I understand David Holmes correctly (on the mailing list), there is a concern that using a per-library-name exclusive lock might not be enough to prevent deadlock with class loading lock. You say: "The deadlock occurs when JNI_OnLoad() loads exactly the same class, whose signature is being verified in another thread." ... Could you explain a little bit more about this scenario? As I understand the JNI_OnLoad() is called as part of native library loading and at that time the thread that is loading native library holds a nativeLibraryLock(libraryName). Now if JNI_OnLoad() triggers loading of a class, whose signature is being verified in another thread (do you mean: which is already being loaded in another thread?) then JNI_OnLoad() thread will block on classLoadingLock(className). But that is not a dead-lock yet. The loading of a class in another thread now has to attempt to load the same native library that is being loaded in the JNI_OnLoad() thread.
So before the proposed patch it was enough for the other thread which was loading the class to trigger loading of any native library, while after the proposed patch, the deadlock would occur only if it triggered loading of the same native library. Am I correctly assessing the problem?

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PR: https://git.openjdk.java.net/jdk/pull/3976