Re: [libvirt] [PATCH v5 2/3] nwfilter: Convert _virNWFilterObjList to use virObjectRWLockable
On 02/08/2018 10:13 PM, Stefan Berger wrote:
On 02/08/2018 08:13 AM, Michal Privoznik wrote:
> On 02/06/2018 08:20 PM, John Ferlan wrote:
>> Implement the self locking object list for nwfilter object lists
>> that uses two hash tables to store the nwfilter object by UUID or
>> by Name.
>>
>> As part of this alter the uuid argument to virNWFilterObjLookupByUUID
>> to expect an already formatted uuidstr.
>>
>> Alter the existing list traversal code to implement the hash table
>> find/lookup/search functionality.
>>
>> Signed-off-by: John Ferlan <jferlan(a)redhat.com>
>> ---
>> src/conf/virnwfilterobj.c | 405
>> ++++++++++++++++++++++++++++-------------
>> src/conf/virnwfilterobj.h | 2 +-
>> src/nwfilter/nwfilter_driver.c | 5 +-
>> 3 files changed, 283 insertions(+), 129 deletions(-)
>
>> @@ -425,24 +483,26 @@
>> virNWFilterObjListAssignDef(virNWFilterObjListPtr nwfilters,
>> return NULL;
>> }
>> -
>> - /* Get a READ lock and immediately promote to WRITE while we adjust
>> - * data within. */
>> - if ((obj = virNWFilterObjListFindByName(nwfilters, def->name))) {
>> + /* We're about to make some changes to objects on the list - so
>> get the
>> + * list READ lock in order to Find the object and WRITE lock the
>> object
>> + * since both paths would immediately promote it anyway */
>> + virObjectRWLockRead(nwfilters);
>> + if ((obj = virNWFilterObjListFindByNameLocked(nwfilters,
>> def->name))) {
>> + virObjectRWLockWrite(obj);
>> + virObjectRWUnlock(nwfilters);
>> objdef = obj->def;
>> if (virNWFilterDefEqual(def, objdef)) {
>> - virNWFilterObjPromoteToWrite(obj);
>> virNWFilterDefFree(objdef);
>> obj->def = def;
>> virNWFilterObjDemoteFromWrite(obj);
>> return obj;
>> }
>> - /* Set newDef and run the trigger with a demoted lock
>> since it may need
>> - * to grab a read lock on this object and promote before
>> returning. */
>> - virNWFilterObjPromoteToWrite(obj);
>> obj->newDef = def;
>> +
>> + /* Demote while the trigger runs since it may need to grab a
>> read
>> + * lock on this object and promote before returning. */
>> virNWFilterObjDemoteFromWrite(obj);
>> /* trigger the update on VMs referencing the filter */
>> @@ -462,39 +522,113 @@
>> virNWFilterObjListAssignDef(virNWFilterObjListPtr nwfilters,
>> return obj;
>> }
>> + /* Promote the nwfilters to add a new object */
>> + virObjectRWUnlock(nwfilters);
>> + virObjectRWLockWrite(nwfilters);
> How can this work? What if there's another thread waiting for the write
> lock (e.g. just to define an NWFilter)? The Unlock(nwfilters) wakes up
> the other thread, it does its job, unlocks the list waking us in turn.
> So we lock @nwfilters for writing and add something into the hash table
> - without all the checks (e.g. duplicity check) done earlier.
Could we keep the read lock and grab a 2nd lock as a write-lock? It
wouldn't be a virObject call, though.
That is not possible because write & read lock must exclude each other
by definition.
Michal