Description of problem:
If multiple domains were being started up in parallel, libvirtd might have deadlocked. 
Because in the child process use unsafe function after fork.

libvirt-3.9.0 function trace:
qemuProcessStart
|_qemuProcessPrepareHostStorage
  |_ ...
    |_virStorageFileGetMetadataRecurse
      |_virStorageFileInitAs
        |_virStorageFileBackendGlusterInit					 
         |_glfs_init (first called)
|_qemuProcessLaunch  
  |_ ...
    |_virProcessRunInMountNamespace (callback : async signal safe functions only)
      |_ virFork
        |_ virProcessNamespaceHelper (child)
          |_virSecurityDACTransactionRun
           |_virSecurityDACSetOwnershipInternal
              |_qemuSecurityChownCallback
                |_virStorageFileInit
                   |_virStorageFileInitAs
                      |_virStorageFileBackendGlusterInit
                         |_glfs_init (bug here: this is not async signal safe 
                           function!! which cause deadlock when muti domains 
                           starting in parallel)
        |__virProcessWait（parent） 


review libvirt-5.10.0 code, seems this problem is not fixed yet:
 virProcessRunInMountNamespace (callback : async signal safe functions only)
  |_virProcessRunInFork
    |_ virFork()
      |_ virProcessRunInForkHelper (child)
        |_ virProcessNamespaceHelper
           |_ virSecurityDACTransactionRun
             |_ ...
               |_ qemuSecurityChownCallback
                 |_virStorageFileInit
                   |_virStorageFileInitAs
                     |_virStorageFileBackendGlusterInit
                         |_glfs_init (bug here: this is not async signal safe 
                           function!! which cause deadlock when muti domains 
                           starting in parallel)

Version-Release number of selected component (if applicable):
CentOS Linux release 7.5.1804
libvirt-3.9.0 
(and libvirt-5.10.0)

How reproducible: 
occasionally

Steps to Reproduce:
1. disk use glusterfs
2. start up domains in parallel

Actual results:
deadlocked.

Expected results:
domains start up success.


Additional info:
[root@CNSZ22PL334 gfs]# pstack 91613
Thread 6 (Thread 0x7ff993ec3700 (LWP 91633)):
#0  0x00007ffa9f30ad42 in pthread_cond_timedwait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
#1  0x00007ffa8c5a03c8 in syncenv_task (proc=proc@entry=0x7ff9fc01ae70) at syncop.c:603
#2  0x00007ffa8c5a12f0 in syncenv_processor (thdata=0x7ff9fc01ae70) at syncop.c:695
#3  0x00007ffa9f306e25 in start_thread () from /lib64/libpthread.so.0
#4  0x00007ffa9f030bad in clone () from /lib64/libc.so.6
Thread 5 (Thread 0x7ff9946c4700 (LWP 91635)):
#0  0x00007ffa9f30ad42 in pthread_cond_timedwait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
#1  0x00007ffa8c5a03c8 in syncenv_task (proc=proc@entry=0x7ff9fc01b230) at syncop.c:603
#2  0x00007ffa8c5a12f0 in syncenv_processor (thdata=0x7ff9fc01b230) at syncop.c:695
#3  0x00007ffa9f306e25 in start_thread () from /lib64/libpthread.so.0
#4  0x00007ffa9f030bad in clone () from /lib64/libc.so.6
Thread 4 (Thread 0x7ff994ec5700 (LWP 91637)):
#0  0x00007ffa9f30df3d in nanosleep () from /lib64/libpthread.so.0
#1  0x00007ffa8c573ba6 in gf_timer_proc (data=0x7ff9fc070a40) at timer.c:164
#2  0x00007ffa9f306e25 in start_thread () from /lib64/libpthread.so.0
#3  0x00007ffa9f030bad in clone () from /lib64/libc.so.6
Thread 3 (Thread 0x7ff9956c6700 (LWP 91638)):
#0  0x00007ffa9f307f97 in pthread_join () from /lib64/libpthread.so.0
#1  0x00007ffa8c5c13c0 in event_dispatch_epoll (event_pool=0x7ff9fc038bf0) at event-epoll.c:746
#2  0x00007ffa8cc74674 in glfs_poller (data=<optimized out>) at glfs.c:672
#3  0x00007ffa9f306e25 in start_thread () from /lib64/libpthread.so.0
#4  0x00007ffa9f030bad in clone () from /lib64/libc.so.6
Thread 2 (Thread 0x7ff9962c7700 (LWP 91639)):
#0  0x00007ffa9f031183 in epoll_wait () from /lib64/libc.so.6
#1  0x00007ffa8c5c0ea2 in event_dispatch_epoll_worker (data=0x7ffa841ea520) at event-epoll.c:649
#2  0x00007ffa9f306e25 in start_thread () from /lib64/libpthread.so.0
#3  0x00007ffa9f030bad in clone () from /lib64/libc.so.6
Thread 1 (Thread 0x7ffa92d6f700 (LWP 91613)):
#0  0x00007ffa9f30d51d in __lll_lock_wait () from /lib64/libpthread.so.0
#1  0x00007ffa9f308e1b in _L_lock_812 () from /lib64/libpthread.so.0
#2  0x00007ffa9f308ce8 in pthread_mutex_lock () from /lib64/libpthread.so.0
#3  0x00007ffa935743d8 in _nss_files_getservbyport_r () from /lib64/libnss_files.so.2
#4  0x00007ffa9f04e9f3 in getservbyport_r@@GLIBC_2.2.5 () from /lib64/libc.so.6
#5  0x00007ffa9f054711 in getnameinfo () from /lib64/libc.so.6
#6  0x00007ffa8c56c121 in gf_resolve_ip6 (hostname=hostname@entry=0x7ff9fc01f920 "10.203.192.226", port=<optimized out>, family=<optimized out>, dnscache=dnscache@entry=0x7ff9fc077438, addr_info=addr_info@entry=0x7ffa92d6ded8) at common-utils.c:332
#7  0x00007ffa3c5e8b22 in af_inet_client_get_remote_sockaddr (sockaddr_len=0x7ffa92d6df68, sockaddr=0x7ffa92d6df80, this=0x7ff9fc0773c0) at name.c:263
#8  socket_client_get_remote_sockaddr (this=this@entry=0x7ff9fc0773c0, sockaddr=sockaddr@entry=0x7ffa92d6df80, sockaddr_len=sockaddr_len@entry=0x7ffa92d6df68, sa_family=sa_family@entry=0x7ffa92d6df66) at name.c:535
#9  0x00007ffa3c5e5608 in socket_connect (this=0x7ff9fc0773c0, port=0) at socket.c:3148
#10 0x00007ffa8ca60de9 in rpc_clnt_reconnect (conn_ptr=0x7ff9fc077040) at rpc-clnt.c:407
#11 0x00007ffa8ca60ea9 in rpc_clnt_start (rpc=rpc@entry=0x7ff9fc077010) at rpc-clnt.c:1196
#12 0x00007ffa8cc78f4d in glfs_mgmt_init (fs=fs@entry=0x7ff9fc07cb10) at glfs-mgmt.c:1032
#13 0x00007ffa8cc748e0 in glfs_volumes_init (fs=fs@entry=0x7ff9fc07cb10) at glfs.c:270
#14 0x00007ffa8cc75c3f in glfs_init_common (fs=0x7ff9fc07cb10) at glfs.c:1039
#15 0x00007ffa8cc75d3f in pub_glfs_init (fs=0x7ff9fc07cb10) at glfs.c:1084
#16 0x00007ffa8ce987ed in virStorageFileBackendGlusterInit (src=0x7ff9fc07d370) at storage/storage_backend_gluster.c:660
#17 0x00007ffa8df08892 in virStorageFileInitAs (src=src@entry=0x7ff9fc07d370, uid=uid@entry=4294967295, gid=gid@entry=4294967295) at storage/storage_source.c:168
#18 0x00007ffa8df0890c in virStorageFileInit (src=src@entry=0x7ff9fc07d370) at storage/storage_source.c:188
#19 0x00007ffa8ba66d53 in qemuSecurityChownCallback (src=<optimized out>, uid=0, gid=0) at qemu/qemu_driver.c:353
#20 0x00007ffaa0c53d32 in virSecurityDACSetOwnershipInternal (priv=0x7ffa8404ec70, src=0x7ffa3000b9c0, path=0x0, uid=0, gid=0) at security/security_dac.c:569
#21 0x00007ffaa0c54cf1 in virSecurityDACTransactionRun (pid=pid@entry=91559, opaque=opaque@entry=0x7ff9fc07d920) at security/security_dac.c:203
#22 0x00007ffaa0aa1d7e in virProcessNamespaceHelper (opaque=0x7ff9fc07d920, cb=0x7ffaa0c54cb0 <virSecurityDACTransactionRun>, pid=91559, errfd=65) at util/virprocess.c:1102
#23 virProcessRunInMountNamespace (pid=pid@entry=91559, cb=cb@entry=0x7ffaa0c54cb0 <virSecurityDACTransactionRun>, opaque=opaque@entry=0x7ff9fc07d920) at util/virprocess.c:1143
#24 0x00007ffaa0c53696 in virSecurityDACTransactionCommit (mgr=<optimized out>, pid=91559) at security/security_dac.c:516
#25 0x00007ffaa0c563e6 in virSecurityManagerTransactionCommit (mgr=0x7ffa8404eca0, pid=pid@entry=91559) at security/security_manager.c:286
#26 0x00007ffaa0c52c43 in virSecurityStackTransactionCommit (mgr=<optimized out>, pid=91559) at security/security_stack.c:166
#27 0x00007ffaa0c563e6 in virSecurityManagerTransactionCommit (mgr=0x7ffa8404f0b0, pid=91559) at security/security_manager.c:286
#28 0x00007ffa8ba7b808 in qemuSecuritySetAllLabel (driver=driver@entry=0x7ffa84016ee0, vm=vm@entry=0x7ffa300028e0, stdin_path=stdin_path@entry=0x0) at qemu/qemu_security.c:54
#29 0x00007ffa8ba0dd13 in qemuProcessLaunch (conn=conn@entry=0x7ff9d8000b80, driver=driver@entry=0x7ffa84016ee0, vm=vm@entry=0x7ffa300028e0, asyncJob=asyncJob@entry=QEMU_ASYNC_JOB_START, incoming=incoming@entry=0x0, snapshot=snapshot@entry=0x0, vmop=vmop@entry=VIR_NETDEV_VPORT_PROFILE_OP_CREATE, flags=flags@entry=17) at qemu/qemu_process.c:5866
#30 0x00007ffa8ba10737 in qemuProcessStart (conn=conn@entry=0x7ff9d8000b80, driver=driver@entry=0x7ffa84016ee0, vm=vm@entry=0x7ffa300028e0, updatedCPU=updatedCPU@entry=0x0, asyncJob=asyncJob@entry=QEMU_ASYNC_JOB_START, migrateFrom=migrateFrom@entry=0x0, migrateFd=migrateFd@entry=-1, migratePath=migratePath@entry=0x0, snapshot=snapshot@entry=0x0, vmop=vmop@entry=VIR_NETDEV_VPORT_PROFILE_OP_CREATE, flags=17, flags@entry=1) at qemu/qemu_process.c:6135
#31 0x00007ffa8ba742d6 in qemuDomainObjStart (conn=0x7ff9d8000b80, driver=driver@entry=0x7ffa84016ee0, vm=0x7ffa300028e0, flags=flags@entry=0, asyncJob=QEMU_ASYNC_JOB_START) at qemu/qemu_driver.c:7283
#32 0x00007ffa8ba74a16 in qemuDomainCreateWithFlags (dom=0x7ff9fc000b00, flags=0) at qemu/qemu_driver.c:7337
#33 0x00007ffaa0b6e9ec in virDomainCreate (domain=domain@entry=0x7ff9fc000b00) at libvirt-domain.c:6531
#34 0x000055dc160fe273 in remoteDispatchDomainCreate (server=0x55dc16b2bf30, msg=0x55dc16b3baa0, args=<optimized out>, rerr=0x7ffa92d6ec50, client=0x55dc16b3c6d0) at remote_dispatch.h:4222
#35 remoteDispatchDomainCreateHelper (server=0x55dc16b2bf30, client=0x55dc16b3c6d0, msg=0x55dc16b3baa0, rerr=0x7ffa92d6ec50, args=<optimized out>, ret=0x7ff9fc000d70) at remote_dispatch.h:4198
#36 0x00007ffaa0bdc682 in virNetServerProgramDispatchCall (msg=0x55dc16b3baa0, client=0x55dc16b3c6d0, server=0x55dc16b2bf30, prog=0x55dc16b37bf0) at rpc/virnetserverprogram.c:437
#37 virNetServerProgramDispatch (prog=0x55dc16b37bf0, server=server@entry=0x55dc16b2bf30, client=0x55dc16b3c6d0, msg=0x55dc16b3baa0) at rpc/virnetserverprogram.c:307
#38 0x000055dc1610eb2d in virNetServerProcessMsg (msg=<optimized out>, prog=<optimized out>, client=<optimized out>, srv=0x55dc16b2bf30) at rpc/virnetserver.c:148
#39 virNetServerHandleJob (jobOpaque=<optimized out>, opaque=0x55dc16b2bf30) at rpc/virnetserver.c:169
#40 0x00007ffaa0ab9851 in virThreadPoolWorker (opaque=opaque@entry=0x55dc16b387e0) at util/virthreadpool.c:167
#41 0x00007ffaa0ab8bd8 in virThreadHelper (data=<optimized out>) at util/virthread.c:206
#42 0x00007ffa9f306e25 in start_thread () from /lib64/libpthread.so.0
#43 0x00007ffa9f030bad in clone () from /lib64/libc.so.6