Using explicit "unlock" causes TimeoutException for other Threads

In a test I have several tasks that run on a single cache node configured as transactional pessimistic replicated cache. If I explicitly call "unlock" then I consistently see TimeoutException reported by some tasks. Without explicit "unlock" the test works fine. Does it mean that I should never call "unlock" and rely on transaction.commit/rollback ? It's seen with infinispan-5.1.2.FINAL

Here's what in a nutshell each task does:

final lockKey = ...

executor.submit(new Callable<Boolean>() {
public Boolean call() throws Exception

TransactionManager tx = cache.getAdvancedCache().getTransactionManager();
tx.begin()
try {
if ( lockManager.lock(lockKey) )

{ // ... // removing next line makes test happy. Otherwise some tasks report TimeoutException (pls. see the stack traces) cache.getLockManager().unlock(lockKey); }

} catch(Throwable t)

{ tx.setRollbackOnly(); }

finally

{ if (ut.getStatus() == Status.STATUS_ACTIVE) ut.commit(); else ut.rollback(); }

By the time I received that exception all other tasks were completed and they released the lock on the key in question.
I also see that LockManagerImpl.lock recognized that the lock was not owned by any thread - see "Lock held by [null]", which seems to be right. But yet the lock failed to be acquired.
Is it a matter to trying it one more time?

org.infinispan.util.concurrent.TimeoutException: Unable to acquire lock after [10 seconds] on key [foo] for requestor [GlobalTransaction:<Sound-15075>:8:local]! Lock held by [null]
at org.infinispan.util.concurrent.locks.LockManagerImpl.lock(LockManagerImpl.java:206)
at org.infinispan.util.concurrent.locks.LockManagerImpl.acquireLock(LockManagerImpl.java:180)
at org.infinispan.util.concurrent.locks.LockManagerImpl.acquireLock(LockManagerImpl.java:170)
at org.infinispan.interceptors.locking.AbstractTxLockingInterceptor.lockKeyAndCheckOwnership(AbstractTxLockingInterceptor.java:209)
at org.infinispan.interceptors.locking.AbstractTxLockingInterceptor.lockAndRegisterBackupLock(AbstractTxLockingInterceptor.java:136)
at org.infinispan.interceptors.locking.PessimisticLockingInterceptor.visitLockControlCommand(PessimisticLockingInterceptor.java:228)
at org.infinispan.commands.control.LockControlCommand.acceptVisitor(LockControlCommand.java:129)
at org.infinispan.interceptors.base.CommandInterceptor.invokeNextInterceptor(CommandInterceptor.java:116)
at org.infinispan.interceptors.base.CommandInterceptor.handleDefault(CommandInterceptor.java:130)
at org.infinispan.commands.AbstractVisitor.visitLockControlCommand(AbstractVisitor.java:159)
at org.infinispan.commands.control.LockControlCommand.acceptVisitor(LockControlCommand.java:129)
at org.infinispan.interceptors.base.CommandInterceptor.invokeNextInterceptor(CommandInterceptor.java:116)
at org.infinispan.interceptors.TxInterceptor.visitLockControlCommand(TxInterceptor.java:144)
at org.infinispan.commands.control.LockControlCommand.acceptVisitor(LockControlCommand.java:129)
at org.infinispan.interceptors.base.CommandInterceptor.invokeNextInterceptor(CommandInterceptor.java:116)
at org.infinispan.interceptors.StateTransferLockInterceptor.handleWithRetries(StateTransferLockInterceptor.java:207)
at org.infinispan.interceptors.StateTransferLockInterceptor.visitLockControlCommand(StateTransferLockInterceptor.java:138)
at org.infinispan.commands.control.LockControlCommand.acceptVisitor(LockControlCommand.java:129)
at org.infinispan.interceptors.base.CommandInterceptor.invokeNextInterceptor(CommandInterceptor.java:116)
at org.infinispan.interceptors.base.CommandInterceptor.handleDefault(CommandInterceptor.java:130)
at org.infinispan.commands.AbstractVisitor.visitLockControlCommand(AbstractVisitor.java:159)
at org.infinispan.commands.control.LockControlCommand.acceptVisitor(LockControlCommand.java:129)
at org.infinispan.interceptors.base.CommandInterceptor.invokeNextInterceptor(CommandInterceptor.java:116)
at org.infinispan.interceptors.InvocationContextInterceptor.handleAll(InvocationContextInterceptor.java:130)
at org.infinispan.interceptors.InvocationContextInterceptor.visitLockControlCommand(InvocationContextInterceptor.java:94)
at org.infinispan.commands.control.LockControlCommand.acceptVisitor(LockControlCommand.java:129)
at org.infinispan.interceptors.InterceptorChain.invoke(InterceptorChain.java:345)
at org.infinispan.CacheImpl.lock(CacheImpl.java:484)
at org.infinispan.CacheImpl.lock(CacheImpl.java:468)