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SubjectRe: [RFC PATCH 02/11] refcount: Implement inc/decrement-and-return functions
On Fri, Sep 01, 2017 at 10:15:39PM +0100, David Howells wrote:
> Peter Zijlstra <peterz@infradead.org> wrote:
>
> > > unsigned int refcount_dec_return(refcount_t *r);
> > > unsigned int refcount_inc_return(refcount_t *r);
> > >
> >
> > I'm not immediately seeing how wanting 1 to mean unused leads to
> > requiring these two functions.
>
> Did you read the other other part of the description?
>
> Further, both functions can be used to accurately trace the refcount
> (refcount_inc() followed by refcount_read() can't be considered
> accurate).

I must admit to having overlooked that. But can we treat the two issues
separately? They are quite distinct.

> > If you'll remember, I did that for inode_count and only needed
> > dec_unless().
>
> I don't remember. inode_count? I can't find such a thing - did you mean
> i_count? I don't find anything matching "dec_unless.*i_count" either.

Ah, yes, i_count. See these:

https://lkml.kernel.org/r/20170224162044.479190330@infradead.org
https://lkml.kernel.org/r/20170224162044.548813302@infradead.org

But looking at them, i_count was rather special, a normal GC based
scheme doesn't need anything new AFAICT:

add:
spin_lock(&map->lock)
refcount_set(&obj->refs, 1);
map_link(map, obj);
spin_unlock(&map->lock);

lookup:
rcu_read_lock();
obj = map_find(map, key);
if (obj && !refcount_inc_not_zero(&obj->refs))
obj = NULL;
rcu_read_unlock();

if (obj) {
/* use obj */
refcount_dec(&obj->refs); /* should never hit 0 */
}

GC:
spin_lock(&map->lock);
map_for_each_obj_safe(obj, map) {
if (refcount_dec_if_one(&obj->refs)) {
map_unlink(map, obj);
rcu_free(obj);
}
}
spin_unlock(&map->lock);

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