[PATCH] virnwfilterobj: Unlock virNWFilterObj before triggering rebuild

[Michal Privoznik]

When updating am NWFilter, rebuilding of all NWFilters is
triggered. This happens in virNWFilterObjListAssignDef() by
calling virNWFilterTriggerRebuild(). Now consider another thread,
that's currently executing virNWFilterBindingCreateXML() over the
same NWFilter.

What happens is that this second thread gets all the way into
virNWFilterInstantiateFilterInternal(), acquires @updateMutex and
transitively calls virNWFilterObjListFindByName() which iterates
over list of NWFilters, locking one by one, comparing names
trying to find the desired one. Sooner or later it will try to
lock the object that the other, original thread is redefining and
which it holds locked. Now that thread can't continue either
because it's waiting for the @updateMutex lock.

So we end up in a typical deadlock situation, one thread holding
lock A trying to acquire lock B, the other thread holding B and
trying to acquire A.

The solution is to unlock the virNWFilterObj in the first thread,
just before triggering rebuild.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=2044379
Signed-off-by: Michal Privoznik <mprivozn at redhat.com>
---

I'm not sure this is 100% correct, but hey - it make that particular bug
go away.

 src/conf/virnwfilterobj.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/src/conf/virnwfilterobj.c b/src/conf/virnwfilterobj.c
index 6bbdf6e6fa..d6c2e59ce8 100644
--- a/src/conf/virnwfilterobj.c
+++ b/src/conf/virnwfilterobj.c
@@ -352,12 +352,17 @@ virNWFilterObjListAssignDef(virNWFilterObjList *nwfilters,
         }
 
         obj->newDef = def;
-        /* trigger the update on VMs referencing the filter */
+
+        /* Trigger the update on VMs referencing the filter, but
+         * unlock the filter because rebuild will lock it again. */
+        virNWFilterObjUnlock(obj);
         if (virNWFilterTriggerRebuild() < 0) {
+            virNWFilterObjLock(obj);
             obj->newDef = NULL;
             virNWFilterObjUnlock(obj);
             return NULL;
         }
+        virNWFilterObjLock(obj);
 
         virNWFilterDefFree(objdef);
         obj->def = def;
-- 
2.34.1