[libvirt] [PATCH 3/3] Avoid rare race when undefining domain

[Martin Kletzander]

When one domain is being undefined and at the same time started, for
example, there is a possibility of a rare problem occuring.

 - Thread 1 does virDomainUndefine(), has the lock, checks that the
   domain is active and because it's not, calls
   virDomainObjListRemove().

 - Thread 2 does virDomainCreate() and tries to lock the domain.

 - Thread 1 needs to lock domain list in order to remove the domain from
   it, but must unlock domain first (proper order is to lock domain list
   first and the domain itself second).

 - Thread 2 grabs the lock, starts the domain and releases the lock.

 - Thread 1 grabs the lock and removes the domain from list.

With this patch:

 - The undefining domain gets marked as "to undefine" before it is
   unlocked.

 - If domain is found in any of the search APIs, it's returned only if
   it is not marked as "to undefine".  The check is done while the
   domain is locked.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1150505

Signed-off-by: Martin Kletzander <mkletzan at redhat.com>
---
 src/conf/domain_conf.c | 23 ++++++++++++++++++++---
 src/conf/domain_conf.h |  1 +
 2 files changed, 21 insertions(+), 3 deletions(-)

diff --git a/src/conf/domain_conf.c b/src/conf/domain_conf.c
index 43574e1..b92a58a 100644
--- a/src/conf/domain_conf.c
+++ b/src/conf/domain_conf.c
@@ -1054,8 +1054,13 @@ virDomainObjPtr virDomainObjListFindByID(virDomainObjListPtr doms,
     virDomainObjPtr obj;
     virObjectLock(doms);
     obj = virHashSearch(doms->objs, virDomainObjListSearchID, &id);
-    if (obj)
+    if (obj) {
         virObjectLock(obj);
+        if (obj->undefining) {
+            virObjectUnlock(obj);
+            obj = NULL;
+        }
+    }
     virObjectUnlock(doms);
     return obj;
 }
@@ -1071,8 +1076,13 @@ virDomainObjPtr virDomainObjListFindByUUID(virDomainObjListPtr doms,
     virUUIDFormat(uuid, uuidstr);

     obj = virHashLookup(doms->objs, uuidstr);
-    if (obj)
+    if (obj) {
         virObjectLock(obj);
+        if (obj->undefining) {
+            virObjectUnlock(obj);
+            obj = NULL;
+        }
+    }
     virObjectUnlock(doms);
     return obj;
 }
@@ -1097,8 +1107,13 @@ virDomainObjPtr virDomainObjListFindByName(virDomainObjListPtr doms,
     virDomainObjPtr obj;
     virObjectLock(doms);
     obj = virHashSearch(doms->objs, virDomainObjListSearchName, name);
-    if (obj)
+    if (obj) {
         virObjectLock(obj);
+        if (obj->undefining) {
+            virObjectUnlock(obj);
+            obj = NULL;
+        }
+    }
     virObjectUnlock(doms);
     return obj;
 }
@@ -2538,6 +2553,7 @@ void virDomainObjListRemove(virDomainObjListPtr doms,

     virUUIDFormat(dom->def->uuid, uuidstr);
     virObjectRef(dom);
+    dom->undefining = true;
     virObjectUnlock(dom);

     VIR_WARN("NOW!!!");
@@ -2563,6 +2579,7 @@ void virDomainObjListRemoveLocked(virDomainObjListPtr doms,
     char uuidstr[VIR_UUID_STRING_BUFLEN];

     virUUIDFormat(dom->def->uuid, uuidstr);
+    dom->undefining = true;
     virObjectUnlock(dom);

     virHashRemoveEntry(doms->objs, uuidstr);
diff --git a/src/conf/domain_conf.h b/src/conf/domain_conf.h
index 9908d88..1d28ed0 100644
--- a/src/conf/domain_conf.h
+++ b/src/conf/domain_conf.h
@@ -2155,6 +2155,7 @@ struct _virDomainObj {
     unsigned int autostart : 1;
     unsigned int persistent : 1;
     unsigned int updated : 1;
+    unsigned int undefining : 1; /* the domain is about to be undefined */

     virDomainDefPtr def; /* The current definition */
     virDomainDefPtr newDef; /* New definition to activate at shutdown */
-- 
2.1.2