RFR: 8266310: deadlock while loading the JNI code

[David Holmes]

Hi Aleksei,

On 11/05/2021 11:19 pm, Aleksei Voitylov 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 ClassLoader per-name locking map has a number of problems so I'm not 
sure using it as a model is a good idea. In particular your new map only 
grows, with entries never being removed AFAICS.

This is a difficult deadlock to solve. Even using a per-entry lock it 
isn't obvious to me that you can't still get a deadlock.

My own thoughts on this problem were that we should not be calling 
JNI_OnLoad with any lock held. But that risks use of a library in a 
separate thread before the JNI+OnLoad has completed. As I said this is a 
difficult deadlock to solve - and may not have a complete solution.

David
-----

> 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.
> 
> -------------
> 
> Commit messages:
>   - JDK-8266310: deadlock while loading the JNI code
>   - JDK-8266310: deadlock while loading the JNI code
> 
> Changes: https://git.openjdk.java.net/jdk/pull/3976/files
>   Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3976&range=00
>    Issue: https://bugs.openjdk.java.net/browse/JDK-8266310
>    Stats: 475 lines in 6 files changed: 456 ins; 0 del; 19 mod
>    Patch: https://git.openjdk.java.net/jdk/pull/3976.diff
>    Fetch: git fetch https://git.openjdk.java.net/jdk pull/3976/head:pull/3976
> 
> PR: https://git.openjdk.java.net/jdk/pull/3976
>