If two threads call executeFront() at the same time, it's possible that none of them executes any task, although there are permits available, and one of the threads just added a task in the queue. This scenario causes random initial state transfer timeouts in the testsuite:

1. T1: continueTaskInBackground(): queue = empty, permits = 0
2. T1: backgroundTaskFinished()
3. T1: = semaphore.release(): queue = empty, permits = 1
4. T1: = executeFront()
5. T1: == semaphore.acquire() -> true: queue = empty, permits = 0
6. T2: == queue.poll() -> null
7. T2: submit(task)
8. T2: = executeFront()
9. T2: == semaphore.acquire() -> false
10. T2: return without executing any task
11. T1: return without executing any task

Failure example: http://ci.infinispan.org/viewLog.html?buildId=27153&tab=buildResultsDiv&buildTypeId=Infinispan_MasterHotspotJdk8#testNameId3522947143330444111