The InitialSync::_fcvFetcherCallback function acquires the InitialSync mutex and then acquires the ReplicationCoordinator mutex. This ordering is inconsistent with some functions in ReplicationCoordinator e.g. ReplicationCoordinatorImpl::processReplSetSyncFrom and ReplicationCoordinatorImpl::processReplSetGetStatus. At the time of writing this, the ReplicationCoordinator mutex acquisition was elimiinated in processReplSetGetStatus by SERVER-34758. The potential deadlock with processReplSetSyncFrom is mostly addressed already in SERVER-28840.

-------------------------

Original Coverity Description:

Threads may try to acquire two locks in different orders, potentially causing deadlock

Defect 103597 (STATIC_C)
Checker ORDER_REVERSAL (subcategory none)
File: /src/mongo/db/repl/initial_syncer.cpp
Function mongo::repl::InitialSyncer::_fcvFetcherCallback(const mongo::StatusWith<mongo::Fetcher::QueryResponse> &, std::shared_ptr<mongo::repl::CallbackCompletionGuard<mongo::StatusWith<mongo::repl::OpTimeWith<long long>>>>, const mongo::repl::OpTimeWith<long long> &)