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[libvirt] [PATCH] virLXCMonitorClose: Unlock domain while closing monitor

There's a race in lxc driver causing a deadlock. If a domain is
destroyed immediately after started, the deadlock can occur. When domain
is started, the even loop tries to connect to the monitor. If the
connecting succeeds, virLXCProcessMonitorInitNotify() is called with
@mon->client locked. The first thing that callee does, is
virObjectLock(vm). So the order of locking is: 1) @mon->client, 2) @vm.

However, if there's another thread executing virDomainDestroy on the
very same domain, the first thing done here is locking the @vm. Then,
the corresponding libvirt_lxc process is killed and monitor is closed
via calling virLXCMonitorClose(). This callee tries to lock @mon->client
too. So the order is reversed to the first case. This situation results
in deadlock and unresponsive libvirtd (since the eventloop is involved).

The proper solution is to unlock the @vm in virLXCMonitorClose prior
entering virNetClientClose(). See the backtrace as follows:

Thread 25 (Thread 0x7f1b7c9b8700 (LWP 16312)):
0  0x00007f1b80539714 in __lll_lock_wait () from /lib64/libpthread.so.0
1  0x00007f1b8053516c in _L_lock_516 () from /lib64/libpthread.so.0
2  0x00007f1b80534fbb in pthread_mutex_lock () from /lib64/libpthread.so.0
3  0x00007f1b82a637cf in virMutexLock (m=0x7f1b3c0038d0) at util/virthreadpthread.c:85
4  0x00007f1b82a4ccf2 in virObjectLock (anyobj=0x7f1b3c0038c0) at util/virobject.c:320
5  0x00007f1b82b861f6 in virNetClientCloseInternal (client=0x7f1b3c0038c0, reason=3) at rpc/virnetclient.c:696
6  0x00007f1b82b862f5 in virNetClientClose (client=0x7f1b3c0038c0) at rpc/virnetclient.c:721
7  0x00007f1b6ee12500 in virLXCMonitorClose (mon=0x7f1b3c007210) at lxc/lxc_monitor.c:216
8  0x00007f1b6ee129f0 in virLXCProcessCleanup (driver=0x7f1b68100240, vm=0x7f1b680ceb70, reason=VIR_DOMAIN_SHUTOFF_DESTROYED) at lxc/lxc_process.c:174
9  0x00007f1b6ee14106 in virLXCProcessStop (driver=0x7f1b68100240, vm=0x7f1b680ceb70, reason=VIR_DOMAIN_SHUTOFF_DESTROYED) at lxc/lxc_process.c:710
10 0x00007f1b6ee1aa36 in lxcDomainDestroyFlags (dom=0x7f1b5c002560, flags=0) at lxc/lxc_driver.c:1291
11 0x00007f1b6ee1ab1a in lxcDomainDestroy (dom=0x7f1b5c002560) at lxc/lxc_driver.c:1321
12 0x00007f1b82b05be5 in virDomainDestroy (domain=0x7f1b5c002560) at libvirt.c:2303
13 0x00007f1b835a7e85 in remoteDispatchDomainDestroy (server=0x7f1b857419d0, client=0x7f1b8574ae40, msg=0x7f1b8574acf0, rerr=0x7f1b7c9b7c30, args=0x7f1b5c004a50) at remote_dispatch.h:3143
14 0x00007f1b835a7d78 in remoteDispatchDomainDestroyHelper (server=0x7f1b857419d0, client=0x7f1b8574ae40, msg=0x7f1b8574acf0, rerr=0x7f1b7c9b7c30, args=0x7f1b5c004a50, ret=0x7f1b5c0029e0) at remote_dispatch.h:3121
15 0x00007f1b82b93704 in virNetServerProgramDispatchCall (prog=0x7f1b8573af90, server=0x7f1b857419d0, client=0x7f1b8574ae40, msg=0x7f1b8574acf0) at rpc/virnetserverprogram.c:435
16 0x00007f1b82b93263 in virNetServerProgramDispatch (prog=0x7f1b8573af90, server=0x7f1b857419d0, client=0x7f1b8574ae40, msg=0x7f1b8574acf0) at rpc/virnetserverprogram.c:305
17 0x00007f1b82b8c0f6 in virNetServerProcessMsg (srv=0x7f1b857419d0, client=0x7f1b8574ae40, prog=0x7f1b8573af90, msg=0x7f1b8574acf0) at rpc/virnetserver.c:163
18 0x00007f1b82b8c1da in virNetServerHandleJob (jobOpaque=0x7f1b8574dca0, opaque=0x7f1b857419d0) at rpc/virnetserver.c:184
19 0x00007f1b82a64158 in virThreadPoolWorker (opaque=0x7f1b8573cb10) at util/virthreadpool.c:144
20 0x00007f1b82a63ae5 in virThreadHelper (data=0x7f1b8574b9f0) at util/virthreadpthread.c:161
21 0x00007f1b80532f4a in start_thread () from /lib64/libpthread.so.0
22 0x00007f1b7fc4f20d in clone () from /lib64/libc.so.6

Thread 1 (Thread 0x7f1b83546740 (LWP 16297)):
0  0x00007f1b80539714 in __lll_lock_wait () from /lib64/libpthread.so.0
1  0x00007f1b8053516c in _L_lock_516 () from /lib64/libpthread.so.0
2  0x00007f1b80534fbb in pthread_mutex_lock () from /lib64/libpthread.so.0
3  0x00007f1b82a637cf in virMutexLock (m=0x7f1b680ceb80) at util/virthreadpthread.c:85
4  0x00007f1b82a4ccf2 in virObjectLock (anyobj=0x7f1b680ceb70) at util/virobject.c:320
5  0x00007f1b6ee13bd7 in virLXCProcessMonitorInitNotify (mon=0x7f1b3c007210, initpid=4832, vm=0x7f1b680ceb70) at lxc/lxc_process.c:601
6  0x00007f1b6ee11fd3 in virLXCMonitorHandleEventInit (prog=0x7f1b3c001f10, client=0x7f1b3c0038c0, evdata=0x7f1b8574a7d0, opaque=0x7f1b3c007210) at lxc/lxc_monitor.c:109
7  0x00007f1b82b8a196 in virNetClientProgramDispatch (prog=0x7f1b3c001f10, client=0x7f1b3c0038c0, msg=0x7f1b3c003928) at rpc/virnetclientprogram.c:259
8  0x00007f1b82b87030 in virNetClientCallDispatchMessage (client=0x7f1b3c0038c0) at rpc/virnetclient.c:1019
9  0x00007f1b82b876bb in virNetClientCallDispatch (client=0x7f1b3c0038c0) at rpc/virnetclient.c:1140
10 0x00007f1b82b87d41 in virNetClientIOHandleInput (client=0x7f1b3c0038c0) at rpc/virnetclient.c:1312
11 0x00007f1b82b88f51 in virNetClientIncomingEvent (sock=0x7f1b3c0044e0, events=1, opaque=0x7f1b3c0038c0) at rpc/virnetclient.c:1832
12 0x00007f1b82b9e1c8 in virNetSocketEventHandle (watch=3321, fd=54, events=1, opaque=0x7f1b3c0044e0) at rpc/virnetsocket.c:1695
13 0x00007f1b82a272cf in virEventPollDispatchHandles (nfds=21, fds=0x7f1b8574ded0) at util/vireventpoll.c:498
14 0x00007f1b82a27af2 in virEventPollRunOnce () at util/vireventpoll.c:645
15 0x00007f1b82a25a61 in virEventRunDefaultImpl () at util/virevent.c:273
16 0x00007f1b82b8e97e in virNetServerRun (srv=0x7f1b857419d0) at rpc/virnetserver.c:1097
17 0x00007f1b8359db6b in main (argc=2, argv=0x7ffff98dbaa8) at libvirtd.c:1512
 src/lxc/lxc_monitor.c | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/src/lxc/lxc_monitor.c b/src/lxc/lxc_monitor.c
index 999039b..07e9301 100644
--- a/src/lxc/lxc_monitor.c
+++ b/src/lxc/lxc_monitor.c
@@ -205,6 +205,9 @@ static void virLXCMonitorDispose(void *opaque)
 void virLXCMonitorClose(virLXCMonitorPtr mon)
+    virDomainObjPtr vm;
+    virNetClientPtr client;
     VIR_DEBUG("mon=%p", mon);
     if (mon->client) {
         /* When manually closing the monitor, we don't
@@ -212,9 +215,18 @@ void virLXCMonitorClose(virLXCMonitorPtr mon)
          * the caller is not re-entrant safe
         VIR_DEBUG("Clear EOF callback mon=%p", mon);
-        mon->cb.eofNotify = NULL;
-        virNetClientClose(mon->client);
-        virObjectUnref(mon->client);
+        vm = mon->vm;
+        client = mon->client;
         mon->client = NULL;
+        mon->cb.eofNotify = NULL;
+        virObjectRef(vm);
+        virObjectUnlock(vm);
+        virNetClientClose(client);
+        virObjectUnref(client);
+        virObjectLock(vm);
+        virObjectUnref(vm);

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