From eeb166cea4a966f01d9241eba1d465d8466e7768 Mon Sep 17 00:00:00 2001
Message-Id: <eeb166cea4a966f01d9241eba1d465d8466e7768@dist-git>
From: Martin Kletzander <mkletzan@redhat.com>
Date: Sat, 13 Dec 2014 10:10:52 +0100
Subject: [PATCH] qemu: avoid rare race when undefining domain

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@redhat.com>
(cherry picked from commit c7d1c139ca3402e875002753952e80ce8054374e)
Signed-off-by: Martin Kletzander <mkletzan@redhat.com>
Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
---
 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 25f20f8..20ae4e7 100644
--- a/src/conf/domain_conf.c
+++ b/src/conf/domain_conf.c
@@ -1056,8 +1056,13 @@ 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;
 }
@@ -1073,8 +1078,13 @@ 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;
 }
@@ -1099,8 +1109,13 @@ 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;
 }
@@ -2537,6 +2552,7 @@ 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 04cee80..01d5aeb 100644
--- a/src/conf/domain_conf.h
+++ b/src/conf/domain_conf.h
@@ -2149,6 +2149,7 @@ 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