[libvirt] [PATCH v2 1/2] conf: Rework virDomainObjListFindByUUID to allow more concurrent APIs

[Martin Kletzander]

Currently, when there is an API that's blocking with locked domain and
second API that's waiting in virDomainObjListFindByUUID() for the domain
lock (with the domain list locked) no other API can be executed on any
domain on the whole hypervisor because all would wait for the domain
list to be locked.  This patch adds new optional approach to this in
which the domain is only ref'd (reference counter is incremented)
instead of being locked and is locked *after* the list itself is
unlocked.  We might consider only ref'ing the domain in the future and
leaving locking on particular APIs, but that's no tonight's fairy tale.

Signed-off-by: Martin Kletzander <mkletzan at redhat.com>
---
v2:
 - rebased on master where searching for domains was changed
 - there's a difference between virDomainObjListFindByUUID and
   virDomainObjListFindByUUIDRef now!

 src/conf/domain_conf.c   | 29 ++++++++++++++++++++++++++---
 src/conf/domain_conf.h   |  2 ++
 src/libvirt_private.syms |  1 +
 3 files changed, 29 insertions(+), 3 deletions(-)

diff --git a/src/conf/domain_conf.c b/src/conf/domain_conf.c
index ba33e62..aafc05e 100644
--- a/src/conf/domain_conf.c
+++ b/src/conf/domain_conf.c
@@ -1074,8 +1074,10 @@ virDomainObjPtr virDomainObjListFindByID(virDomainObjListPtr doms,
 }


-virDomainObjPtr virDomainObjListFindByUUID(virDomainObjListPtr doms,
-                                           const unsigned char *uuid)
+static virDomainObjPtr
+virDomainObjListFindByUUIDInternal(virDomainObjListPtr doms,
+                                   const unsigned char *uuid,
+                                   bool ref)
 {
     char uuidstr[VIR_UUID_STRING_BUFLEN];
     virDomainObjPtr obj;
@@ -1084,17 +1086,38 @@ virDomainObjPtr virDomainObjListFindByUUID(virDomainObjListPtr doms,
     virUUIDFormat(uuid, uuidstr);

     obj = virHashLookup(doms->objs, uuidstr);
+    if (ref) {
+        virObjectRef(obj);
+        virObjectUnlock(doms);
+    }
     if (obj) {
         virObjectLock(obj);
         if (obj->removing) {
+            if (ref)
+                virObjectUnref(obj);
             virObjectUnlock(obj);
             obj = NULL;
         }
     }
-    virObjectUnlock(doms);
+    if (!ref)
+        virObjectUnlock(doms);
     return obj;
 }

+virDomainObjPtr
+virDomainObjListFindByUUID(virDomainObjListPtr doms,
+                           const unsigned char *uuid)
+{
+    return virDomainObjListFindByUUIDInternal(doms, uuid, false);
+}
+
+virDomainObjPtr
+virDomainObjListFindByUUIDRef(virDomainObjListPtr doms,
+                              const unsigned char *uuid)
+{
+    return virDomainObjListFindByUUIDInternal(doms, uuid, true);
+}
+
 static int virDomainObjListSearchName(const void *payload,
                                       const void *name ATTRIBUTE_UNUSED,
                                       const void *data)
diff --git a/src/conf/domain_conf.h b/src/conf/domain_conf.h
index 75e3d2a..57297cd 100644
--- a/src/conf/domain_conf.h
+++ b/src/conf/domain_conf.h
@@ -2287,6 +2287,8 @@ virDomainObjPtr virDomainObjListFindByID(virDomainObjListPtr doms,
                                          int id);
 virDomainObjPtr virDomainObjListFindByUUID(virDomainObjListPtr doms,
                                            const unsigned char *uuid);
+virDomainObjPtr virDomainObjListFindByUUIDRef(virDomainObjListPtr doms,
+                                              const unsigned char *uuid);
 virDomainObjPtr virDomainObjListFindByName(virDomainObjListPtr doms,
                                            const char *name);

diff --git a/src/libvirt_private.syms b/src/libvirt_private.syms
index 57cf4d3..613fb79 100644
--- a/src/libvirt_private.syms
+++ b/src/libvirt_private.syms
@@ -353,6 +353,7 @@ virDomainObjListExport;
 virDomainObjListFindByID;
 virDomainObjListFindByName;
 virDomainObjListFindByUUID;
+virDomainObjListFindByUUIDRef;
 virDomainObjListForEach;
 virDomainObjListGetActiveIDs;
 virDomainObjListGetInactiveNames;
--
2.2.0