On Fri, Sep 01, 2017 at 10:15:39PM +0100, David Howells wrote: > Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx https://lkml.kernel.org/r/20170224162044.548813302@xxxxxxxxxxxxx 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);
