[libvirt] [PATCH v3] qemu: 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>
---

Notes:
    I marked this as v3, but it is basically a resend of v1 [1] with
    s/undefining/removing/ change.  I'm sending this even though the second
    version of the patch got in again because the v2 that we pushed didn't
    help.  I got convinced [2] that doing this with a job fixes it, but it
    doesn't, it just serializes the Create API after the Undefine API.  The
    only difference that the commit b629c64e [3] does is that it waits on
    obtaining job, but not on locking the domain.  That previous commit can
    be reverted if anyone wants, I see almost no point in it being there
    now.
    
    [1] http://www.redhat.com/archives/libvir-list/2014-October/msg01055.html
    [2] http://www.redhat.com/archives/libvir-list/2014-October/msg01115.html
    [3] http://libvirt.org/git/?p=libvirt.git;a=commitdiff;h=b629c64e

 src/conf/domain_conf.c | 22 +++++++++++++++++++---
 src/conf/domain_conf.h |  1 +
 2 files changed, 20 insertions(+), 3 deletions(-)

diff --git a/src/conf/domain_conf.c b/src/conf/domain_conf.c
index ec45b8c..8e08407 100644
--- a/src/conf/domain_conf.c
+++ b/src/conf/domain_conf.c
@@ -1062,8 +1062,13 @@ virDomainObjPtr virDomainObjListFindByID(virDomainObjListPtr doms,
     virDomainObjPtr obj;
     virObjectLock(doms);
     obj = virHashSearch(doms->objs, virDomainObjListSearchID, &id);
-    if (obj)
+    if (obj) {
         virObjectLock(obj);
+        if (obj->removing) {
+            virObjectUnlock(obj);
+            obj = NULL;
+        }
+    }
     virObjectUnlock(doms);
     return obj;
 }
@@ -1079,8 +1084,13 @@ virDomainObjPtr virDomainObjListFindByUUID(virDomainObjListPtr doms,
     virUUIDFormat(uuid, uuidstr);

     obj = virHashLookup(doms->objs, uuidstr);
-    if (obj)
+    if (obj) {
         virObjectLock(obj);
+        if (obj->removing) {
+            virObjectUnlock(obj);
+            obj = NULL;
+        }
+    }
     virObjectUnlock(doms);
     return obj;
 }
@@ -1105,8 +1115,13 @@ virDomainObjPtr virDomainObjListFindByName(virDomainObjListPtr doms,
     virDomainObjPtr obj;
     virObjectLock(doms);
     obj = virHashSearch(doms->objs, virDomainObjListSearchName, name);
-    if (obj)
+    if (obj) {
         virObjectLock(obj);
+        if (obj->removing) {
+            virObjectUnlock(obj);
+            obj = NULL;
+        }
+    }
     virObjectUnlock(doms);
     return obj;
 }
@@ -2551,6 +2566,7 @@ void virDomainObjListRemove(virDomainObjListPtr doms,
 {
     char uuidstr[VIR_UUID_STRING_BUFLEN];

+    dom->removing = true;
     virUUIDFormat(dom->def->uuid, uuidstr);
     virObjectRef(dom);
     virObjectUnlock(dom);
diff --git a/src/conf/domain_conf.h b/src/conf/domain_conf.h
index e10b3c5..75e3d2a 100644
--- a/src/conf/domain_conf.h
+++ b/src/conf/domain_conf.h
@@ -2187,6 +2187,7 @@ struct _virDomainObj {
     unsigned int autostart : 1;
     unsigned int persistent : 1;
     unsigned int updated : 1;
+    unsigned int removing : 1;

     virDomainDefPtr def; /* The current definition */
     virDomainDefPtr newDef; /* New definition to activate at shutdown */
-- 
2.2.0