Re: reference counts

[Paul Goyette <paul%vps1.whooppee.com@localhost>]

Hmmm.....

> static inline bool
> refcount_dec_local(struct refcount *refcount)
> {
>         unsigned old, new;
>
>         do {
>                 old = refcount->rc_value;
>                 if (old == 1) {
>                         /*
>                          * Avoid an atomic if we don't need it. Caller
>                          * guarantees that if the reference count is 1,
>                          * nobody else can acquire new references.
>                          */
>                         refcount->rc_value = 0;
>                         return true;
>                 }
>                 KASSERT(0 < old);
>                 new = old - 1;
>         } while (atomic_cas_uint(&refcount->rc_value, old, new) != old);
>
>         KASSERT(0 < new);
>         return false;
> }

Maybe I'm just seeing this clearly, but if old == 1, and new = old - 1, 
doesn't that mean new = 0?  And if so, the guarantee that "if the 
reference count is 1 ..." becomes a guarantee to KASSERT(0 < new) ?

Maybe the KASSERT should be (0 <= old) ??







On Sat, 11 Apr 2015, Taylor R Campbell wrote:

> I would like to introduce an easy-to-use-correctly abstraction for
> MP-safe reference counts.  I proposed the attached API, based on many
> examples and on discussion with dholland@.
>
> It is designed to address three different use cases:
>
> - Object with lifetime ended by a specific operation which needs to
> wait until all users are done.
>
> Example: autoconf device with cdevsw nodes.  xyz_detach must wait for
> all open files to be closed, and xyz_open/xyz_close must maintain the
> reference count and notify xyz_detach when the last close happens.
> (We don't exactly do this currently, but that's a bug.)
>
> - Object in global cache with last-user-frees.
>
> Example: vnodes in vcache.  Last reference must be dropped under the
> same lock as removing the vnode from the vcache or at least marking it
> unusable.  Otherwise, maintenance of the vnode reference count can be
> done with atomic operations.
>
> - Shared objects, not in any global tables, with last-user-frees.
>
> Example: process credentials.  Last user just calls kmem_free, no
> table or locks involved.
>
> Thoughts?

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