RFR: 8266310: deadlock while loading the JNI code [v2]

[Peter Levart]

Hi Aleksei,

Are you trying to solve this in principle or do you have a concrete 
problem at hand which triggers this deadlock? If it is the later, then 
some rearrangement of code might do the trick... For example, native 
libraries are typically loaded by a class initializer of some class that 
is guaranteed to be initialized before the 1st invocation of a native 
method from such library. But if such class can also be loaded and 
initialized by some other trigger, deadlock can occur. Best remedy for 
such situation is to move all native methods to a special class that 
serves just for interfacing with native code and also contains an 
initializer that loads the native library and nothing else. Such 
arrangement would ensure that the order of taking locks is always the 
same: classLoadingLock -> nativeLibraryLock ...

Regards, Peter

On 5/20/21 12:31 AM, David Holmes wrote:
> On 20/05/2021 2:29 am, Aleksei Voitylov wrote:
>> On Wed, 19 May 2021 16:21:41 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.
>>>
>>> Aleksei Voitylov has updated the pull request incrementally with one 
>>> additional commit since the last revision:
>>>
>>>    address review comments, add tests
>>
>> Dear colleagues,
>>
>> The updated PR addresses review comment regarding ThreadLocal as well 
>> as David' concern around the lock being held during 
>> JNI_OnLoad/JNI_OnUnload calls, and ensures all lock objects are 
>> deallocated. Multiple threads are allowed to enter 
>> NativeLibrary.load() to prevent any thread from locking while another 
>> thread loads a library. Before the update, there could be a class 
>> loading lock held by a parallel capable class loader, which can 
>> deadlock with the library loading lock. As proposed by David Holmes, 
>> the library loading lock was removed because dlopen/LoadLibrary are 
>> thread safe and they maintain internal reference counters on 
>> libraries. There's still a lock being held while a pair of containers 
>> are read/updated. It's not going to deadlock as there's no lock/wait 
>> operation performed while that lock is held. Multiple threads may 
>> create their own copies of NativeLibrary object and register it for 
>> auto unloading.
>>
>> Tests for auto unloading were added along with the PR update. There 
>> are now 3 jtreg tests:
>> - one checks for deadlock, similar to the one proposed by Chris Hegarty
>> - two other tests are for library unload.
>>
>> The major side effect of that multiple threads are allowed to enter 
>> is that JNI_OnLoad/JNI_OnUnload may be called multiple (but same) 
>> number of times from concurrent threads. In particular, the number of 
>> calls to JNI_OnLoad must be equal to the number of calls to 
>> JNI_OnUnload after the relevant class loader is garbage collected. 
>> This may affect the behaviour that relies on specific order or the 
>> number of JNI_OnLoad/JNI_OnUnload calls. The current JNI 
>> specification does not mandate how many times JNI_OnLoad/JNI_OnUnload 
>> are called. Also, we could not locate tests in jck/jtreg/vmTestbase 
>> that would rely on the specific order or number of calls to 
>> JNI_OnLoad/JNI_OnUnload.
>
> But you can't make such a change! That was my point. To fix the 
> deadlock we must not hold a lock. But we must ensure only a single 
> call to JNI_OnLoad is possible. It is an unsolvable problem with those 
> constraints. You can't just change the behaviour of JNI_OnLoad like that.
>
> David
> -----
>

If this is really a problem that several people are facing, then perhaps 
a change in the API could solve it. I'm thinking

>> Thank you Alan Bateman, David Holmes and Chris Hegarty for your 
>> valuable input.
>>
>> -------------
>>
>> PR: https://git.openjdk.java.net/jdk/pull/3976
>>