[libvirt] [PATCH] lxc_monitor: Avoid AB / BA lock race

[Mark Asselstine]

A deadlock situation can occur when autostarting a LXC domain 'guest'
due to two threads attempting to take opposing locks while holding
opposing locks (AB BA problem). Thread A takes and holds the 'vm' lock
while attempting to take the 'client' lock, meanwhile, thread B takes
and holds the 'client' lock while attempting to take the 'vm' lock.

The potential for this can be seen as follows:

Thread A:
virLXCProcessAutostartDomain (takes vm lock)
 --> virLXCProcessStart
  --> virLXCProcessConnectMonitor
   --> virLXCMonitorNew
    --> virNetClientSetCloseCallback (wants client lock)

Thread B:
virNetClientIncomingEvent (takes client lock)
 --> virNetClientIOHandleInput
  --> virNetClientCallDispatch
   --> virNetClientCallDispatchMessage
    --> virNetClientProgramDispatch
     --> virLXCMonitorHandleEventInit
      --> virLXCProcessMonitorInitNotify (wants vm lock)

Since these threads are scheduled independently and are preemptible it
is possible for the deadlock scenario to occur where each thread locks
their first lock but both will fail to get their second lock and just
spin forever. You get something like:

virLXCProcessAutostartDomain (takes vm lock)
 --> virLXCProcessStart
  --> virLXCProcessConnectMonitor
   --> virLXCMonitorNew
<...>
virNetClientIncomingEvent (takes client lock)
 --> virNetClientIOHandleInput
  --> virNetClientCallDispatch
   --> virNetClientCallDispatchMessage
    --> virNetClientProgramDispatch
     --> virLXCMonitorHandleEventInit
      --> virLXCProcessMonitorInitNotify (wants vm lock but spins)
<...>
    --> virNetClientSetCloseCallback (wants client lock but spins)

Neither thread ever gets the lock it needs to be able to continue
while holding the lock that the other thread needs.

The actual window for preemption which can cause this deadlock is
rather small, between the calls to virNetClientProgramNew() and
execution of virNetClientSetCloseCallback(), both in
virLXCMonitorNew(). But it can be seen in real world use that this
small window is enough.

By moving the call to virNetClientSetCloseCallback() ahead of
virNetClientProgramNew() we can close any possible chance of the
deadlock taking place. There should be no other implications to the
move since the close callback (in the unlikely event was called) will
spin on the vm lock. The remaining work that takes place between the
old call location of virNetClientSetCloseCallback() and the new
location is unaffected by the move.

Signed-off-by: Mark Asselstine <mark.asselstine at windriver.com>
---

NOTE I am not intimately familiar with the libvirt code, so although I
am confident of the cause of the issue I am not sure this is the best
solution. So I am really curious what people who do know this code
think and as such I am definitely open to discussions and suggestions
on how to rework things to properly address the issue.

 src/lxc/lxc_monitor.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/src/lxc/lxc_monitor.c b/src/lxc/lxc_monitor.c
index e765c16..218f426 100644
--- a/src/lxc/lxc_monitor.c
+++ b/src/lxc/lxc_monitor.c
@@ -161,6 +161,10 @@ virLXCMonitorPtr virLXCMonitorNew(virDomainObjPtr vm,
     if (virNetClientRegisterAsyncIO(mon->client) < 0)
         goto error;
 
+    /* avoid deadlock by making this call before assigning virLXCMonitorEvents */
+    virNetClientSetCloseCallback(mon->client, virLXCMonitorEOFNotify, mon,
+                                 virLXCMonitorCloseFreeCallback);
+
     if (!(mon->program = virNetClientProgramNew(VIR_LXC_MONITOR_PROGRAM,
                                                 VIR_LXC_MONITOR_PROGRAM_VERSION,
                                                 virLXCMonitorEvents,
@@ -176,8 +180,6 @@ virLXCMonitorPtr virLXCMonitorNew(virDomainObjPtr vm,
     memcpy(&mon->cb, cb, sizeof(mon->cb));
 
     virObjectRef(mon);
-    virNetClientSetCloseCallback(mon->client, virLXCMonitorEOFNotify, mon,
-                                 virLXCMonitorCloseFreeCallback);
 
  cleanup:
     VIR_FREE(sockpath);
-- 
2.7.4