when create index with background on one database, the database x lock held for a long time, with the help of gdb, i found it was blocked by the wiredtiger's checkpoint mutex, the gdb stack is:

Thread 145 (Thread 0x7fab66ff4700 (LWP 1426590)):
#0 __lll_lock_wait () at ../nptl/sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:135
#1 0x00007fab8e9cc479 in _L_lock_909 () from /lib/x86_64-linux-gnu/libpthread.so.0
#2 0x00007fab8e9cc2a0 in _GI__pthread_mutex_lock (mutex=0x55620d9a7538) at ../nptl/pthread_mutex_lock.c:79
#3 0x0000556208d4180b in __session_alter ()
#4 0x0000556208cdb5c5 in mongo::WiredTigerUtil::setTableLogging(_wt_session*, std::_cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, bo
ol) ()
#5 0x0000556208cdbb58 in mongo::WiredTigerUtil::setTableLogging(mongo::OperationContext*, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >
const&, bool) ()
#6 0x0000556208ca6ea4 in mongo::WiredTigerIndex::WiredTigerIndex(mongo::OperationContext*, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>
> const&, mongo::IndexDescriptor const*, mongo::KVPrefix, bool) ()
#7 0x0000556208ca7125 in mongo::WiredTigerIndexStandard::WiredTigerIndexStandard(mongo::OperationContext*, std::__cxx11::basic_string<char, std::char_traits<char>, std::
allocator<char> > const&, mongo::IndexDescriptor const*, mongo::KVPrefix, bool) ()
#8 0x0000556208cae9a7 in mongo::WiredTigerKVEngine::getGroupedSortedDataInterface(mongo::OperationContext*, mongo::StringData, mongo::IndexDescriptor const*, mongo::KVPr
efix) ()
#9 0x0000556208e2ab4d in mongo::KVDatabaseCatalogEntry::getIndex(mongo::OperationContext*, mongo::CollectionCatalogEntry const*, mongo::IndexCatalogEntry*) ()
#10 0x000055620936b550 in mongo::IndexCatalogImpl::_setupInMemoryStructures(mongo::OperationContext*, std::unique_ptr<mongo::IndexDescriptor, std::default_delete<mongo::I
ndexDescriptor> >, bool) ()
#11 0x0000556209d21a9e in mongo::IndexCatalog::_setupInMemoryStructures(mongo::OperationContext*, std::unique_ptr<mongo::IndexDescriptor, std::default_delete<mongo::Index
Descriptor> >, bool) ()
#12 0x0000556209364029 in mongo::IndexCatalogImpl::IndexBuildBlock::init() ()
#13 0x0000556209374b2e in mongo::MultiIndexBlockImpl::init(std::vector<mongo::BSONObj, std::allocator<mongo::BSONObj> > const&) ()
#14 0x000055620925cb0f in auto mongo::writeConflictRetry<mongo::CmdCreateIndex::errmsgRun(mongo::OperationContext*, std::_cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, mongo::BSONObj const&, std::cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&, mongo::BSONObjBuilder&)::{lambda()#6}>(mongo::OperationContext*, mongo::StringData, mongo::CmdCreateIndex::errmsgRun(mongo::OperationContext*, std::cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, mongo::BSONObj const&, std::cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&, mongo::BSONObjBuilder&)::{lambda()#6}, mongo::CmdCreateIndex::errmsgRun(mongo::OperationContext*, std::cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, mongo::BSONObj const&, std::_cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&, mongo::BSONObjBuilder&)::{lambda()#6}&&) ()