Re: [PATCH rdma-next v6 3/8] RDMA/restrack: Add general infrastructure to track RDMA resources

[Jason Gunthorpe <jgg-uk2M96/98Pc@public.gmane.org>]

On Thu, Jan 25, 2018 at 10:27:10PM +0200, Leon Romanovsky wrote:
> On Thu, Jan 25, 2018 at 01:10:23PM -0700, Jason Gunthorpe wrote:
> > On Thu, Jan 25, 2018 at 09:44:36PM +0200, Leon Romanovsky wrote:
> > > > > +	/*
> > > > > +	 * @kref: Protect destroy of the resource
> > > > > +	 */
> > > > > +	struct kref		kref;
> > > >
> > > > Sticking a kref in a random structure that is itself not malloc'd is a
> > > > big red flag:
> > >
> > > It is not "rand", but embedded part of ib_qp,ib_pd and ib_cq. It is
> > > malloced as part of their creation.
> >
> > I mean the kref isn't in any way linked the lifetime of the malloc
> > that contains it. So it isn't really a kref, it is just a refcount.
> 
> For now, yes, in the future no. IMHO it is the direction to manage RDMA
> objects.

Maybe but until we do that this doesn't have struct kref semantics at
all and shouldn't be called kref..

> > What I sent you wasn't remotely like this, it had two nested locks,
> > the outer mutex and a very special open coded inner read/write lock
> > that doesn't deadlock when nested..
> 
> I still didn't like your approach, because it has three words which i don't want
> to see here: "special open-coded variant". I believe that the way to go is to add
> completion structure and convert _del to be something like that:

Well, we can code the same idea with a completion, it is a little
clearer but uses more memory for the per-object completion struct.

Read side:
  mutex_lock(list_lock)
  for_each_list(...,obj...) { // Must NOT use _safe here
     // The object is in process of being deleted
     if (refcount_inc_not_zero(&obj->refcount))
          continue;
     mutex_unlock(list_lock);
      ...
     mutex_lock(list_lock);
     if (refcount_dec_and_test(&obj->recount))
         complete(&res->completion);
  }

Destroy side:
   if (refcount_dec_and_test(&obj->ref))
         complete(&obj->completion);
   wait_for_completion(obj->free);

   mutex_lock(list_lock)
   list_del(obj);


The refcount starts at 1 during init. Destroy triggers the freeing
process by decr. Once the refcount reaches 0 it latches at 0 as 'to be
destroyed' and the pointer can never leave the list_lock critical
section.

It is still tricky..

Jason
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