[libvirt] [PATCH v5 1/3] nwfilter: Convert _virNWFilterObj to use virObjectRWLockable

[Michal Privoznik]

On 02/06/2018 08:20 PM, John Ferlan wrote:
> Unlike it's counterparts, the nwfilter object code needs to be able
> to support recursive read locks while processing and checking new
> filters against the existing environment. Thus instead of using a
> virObjectLockable which uses pure mutexes, use the virObjectRWLockable
> and primarily use RWLockRead when obtaining the object lock since
> RWLockRead locks can be recursively obtained (up to a limit) as long
> as there's a corresponding unlock.
> 
> Since all the object management is within the virnwfilterobj code, we
> can limit the number of Write locks on the object to very small areas
> of code to ensure we don't run into deadlock's with other threads and
> domain code that will check/use the filters (it's a very delicate
> balance). This limits the write locks to AssignDef and Remove processing.
> 
> This patch will introduce a new API virNWFilterObjEndAPI to unlock
> and deref the object.
> 
> This patch introduces two helpers to promote/demote the read/write lock.
> 
> Signed-off-by: John Ferlan <jferlan at redhat.com>
> ---
>  src/conf/virnwfilterobj.c              | 193 +++++++++++++++++++++++----------
>  src/conf/virnwfilterobj.h              |   9 +-
>  src/libvirt_private.syms               |   3 +-
>  src/nwfilter/nwfilter_driver.c         |  15 +--
>  src/nwfilter/nwfilter_gentech_driver.c |  11 +-
>  5 files changed, 153 insertions(+), 78 deletions(-)
> 
> diff --git a/src/conf/virnwfilterobj.c b/src/conf/virnwfilterobj.c
> index 87d7e7270..cd52706ec 100644
> --- a/src/conf/virnwfilterobj.c
> +++ b/src/conf/virnwfilterobj.c
> @@ -34,7 +34,7 @@
>  VIR_LOG_INIT("conf.virnwfilterobj");
>  
>  struct _virNWFilterObj {
> -    virMutex lock;
> +    virObjectRWLockable parent;
>  
>      bool wantRemoved;
>  
> @@ -47,27 +47,69 @@ struct _virNWFilterObjList {
>      virNWFilterObjPtr *objs;
>  };
>  
> +static virClassPtr virNWFilterObjClass;
> +static void virNWFilterObjDispose(void *opaque);
> +
> +
> +static int
> +virNWFilterObjOnceInit(void)
> +{
> +    if (!(virNWFilterObjClass = virClassNew(virClassForObjectRWLockable(),
> +                                            "virNWFilterObj",
> +                                            sizeof(virNWFilterObj),
> +                                            virNWFilterObjDispose)))
> +        return -1;
> +
> +    return 0;
> +}
> +
> +VIR_ONCE_GLOBAL_INIT(virNWFilterObj)
> +
>  
>  static virNWFilterObjPtr
>  virNWFilterObjNew(void)
>  {
>      virNWFilterObjPtr obj;
>  
> -    if (VIR_ALLOC(obj) < 0)
> +    if (virNWFilterObjInitialize() < 0)
>          return NULL;
>  
> -    if (virMutexInitRecursive(&obj->lock) < 0) {
> -        virReportError(VIR_ERR_INTERNAL_ERROR,
> -                       "%s", _("cannot initialize mutex"));
> -        VIR_FREE(obj);
> +    if (!(obj = virObjectRWLockableNew(virNWFilterObjClass)))
>          return NULL;
> -    }
>  
> -    virNWFilterObjLock(obj);
> +    virObjectRWLockWrite(obj);
>      return obj;
>  }
>  
>  
> +static void
> +virNWFilterObjPromoteToWrite(virNWFilterObjPtr obj)
> +{
> +    virObjectRWUnlock(obj);
> +    virObjectRWLockWrite(obj);
> +}

How can this not deadlock? This will work only if @obj is locked exactly
once. But since we allow recursive locks any lock() that happens in the
2nd level must deadlock with this. On the other hand, there's no such
case in the code currently. But that doesn't stop anybody from calling
PromoteWrite() without understanding its limitations.

Maybe the fact that we need recursive lock for NWFilterObj means it's
not a good candidate for virObjectRWLocable? It is still a good
candidate for virObject though.

Or if you want to spend extra time on this, maybe introducing
virObjectRecursiveLockable may be worth it (terrible name, I know).


> @@ -347,26 +426,39 @@ virNWFilterObjListAssignDef(virNWFilterObjListPtr nwfilters,
>      }
>  
>  
> +    /* Get a READ lock and immediately promote to WRITE while we adjust
> +     * data within. */
>      if ((obj = virNWFilterObjListFindByName(nwfilters, def->name))) {
>  
>          objdef = obj->def;
>          if (virNWFilterDefEqual(def, objdef)) {
> +            virNWFilterObjPromoteToWrite(obj);
>              virNWFilterDefFree(objdef);
>              obj->def = def;
> +            virNWFilterObjDemoteFromWrite(obj);
>              return obj;
>          }

What is the idea behind this if()? I don't understand it. There doesn't
seem to be any fields in @objdef or

>  
> +        /* 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;
> +        virNWFilterObjDemoteFromWrite(obj);
> +
>          /* trigger the update on VMs referencing the filter */
>          if (virNWFilterTriggerVMFilterRebuild() < 0) {
> +            virNWFilterObjPromoteToWrite(obj);
>              obj->newDef = NULL;
> -            virNWFilterObjUnlock(obj);
> +            /* NB: We're done, no need to Demote and End, just End */
> +            virNWFilterObjEndAPI(&obj);
>              return NULL;
>          }
>  
> +        virNWFilterObjPromoteToWrite(obj);
>          virNWFilterDefFree(objdef);
>          obj->def = def;
>          obj->newDef = NULL;
> +        virNWFilterObjDemoteFromWrite(obj);
>          return obj;
>      }
>  

Michal