RFR: 8266310: deadlock while loading the JNI code

[Aleksei Voitylov]

Hi David,

On 12/05/2021 10:56, David Holmes wrote:
> 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.
OK. I'm working on a version of a patch which addresses this concern.
>
> 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.

I believe the patch solves the deadlock when different libraries are in
play. Yes, it's still possible to get a deadlock when a library with a
JNI_Onload instantiates a class that requires the same library to be
loaded. Is this a valid use case to begin with for future use and why
such a recursion should be allowed? If not, I'd like to file a CSR for
JNI specification with the intent to forbid such behavior. It's obvious
there are no current users of such behavior because it deadlocks.

>
> 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.

Thanks, this is a very appealing idea. While it's true dlopen on Linux
and Mac OS are explicitly marked thread safe (and LoadLibrary on Windows
can probably be assumed so as well, though is not marked so in Microsoft
documentation), all implementations use reference counting techniques,
and the number of calls to open has to be equal to the number of calls
to close. I don't know how to achieve that without locks.

-Aleksei

>
> 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
>>