Re: Lockless uidinfo.

* Pawel Jakub Dawidek <pjd@xxxxxxxxxxx> [070818 09:14] wrote:
On Sat, Aug 18, 2007 at 08:50:41AM -0700, Alfred Perlstein wrote:
* Pawel Jakub Dawidek <pjd@xxxxxxxxxxx> [070818 07:59] wrote:
Yes, to lookup uidinfo you need to hold uihashtbl_mtx mutex, so once you
hold it and ui_ref is 0, noone will be able to reference it, because it
has to wait to look it up.

And the field doesn't need to be volatile to prevent cached/opportunitic
reads?

The only chance of something like this will be the scenario below:

thread1 (uifind) thread2 (uifree)
---------------- ----------------
refcount_release(&uip->ui_ref))
/* ui_ref == 0 */
mtx_lock(&uihashtbl_mtx);
refcount_acquire(&uip->ui_ref);
/* ui_ref == 1 */
mtx_unlock(&uihashtbl_mtx);
mtx_lock(&uihashtbl_mtx);
if (uip->ui_ref > 0) {
mtx_unlock(&uihashtbl_mtx);
return;
}

Now, you suggest that ui_ref in 'if (uip->ui_ref > 0)' may still have
cached 0? I don't think it is possible, first refcount_acquire() uses
read memory bariers (but we may still need ui_ref to volatile for this
to make any difference) and second, think of ui_ref as a field protected
by uihashtbl_mtx mutex in this very case.

Is my thinking correct?

I don't know, that's why I was asking you. :)

--
- Alfred Perlstein
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