On Wed, Jan 15, 2020 at 09:24:28AM -0400, Jason Gunthorpe wrote:
> > Your requirement seems a little different, and in fact in many ways
> > similar to the percpu_ref primitive.
> I was interested because you are talking about allowing the read/write side
> of a rw sem to be held across a return to user space/etc, which is the
> same basic problem.
> precpu refcount looks more like a typical refcount with a release that
> is called by whatever context does the final put. The point above is
> to basically move the release of a refcount into a synchrnous path by
> introducing some barrier to wait for the refcount to go to zero. In
> the above the barrier is the down_write() as it is really closer to a
> rwsem than a refcount.

No, percpu_ref is a little different than the name suggests, as it has
a magic initial reference, and then the other short term reference.  To
actually tear it down now just a normal put of the reference is needed,
but an explicit percpu_ref_kill or percpu_ref_kill_and_confirm.  Various
callers (including all I added) would like that operation to be
synchronous and currently hack that up, so a version of the percpu_ref
that actually waits for the other references to away like we hacked
up various places seems to exactly suit your requirements.

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