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>>>> + >>>> + if (refcount_read(xa_load(&irqs, info->irq)) > 1) >>> >>> refcount combined with xa? That feels wrong, why is refcount used for >>> this at all? >> >> Not long ago I commented on similar usage for ice driver, >> ~"since you are locking anyway this could be a plain counter", >> and author replied >> ~"additional semantics (like saturation) of refcount make me feel warm >> and fuzzy" (sorry if misquoting too much). >> That convinced me back then, so I kept quiet about that here. > > But why is this being incremented / decremented at all? What is that > for? [global] This is just a counter, it is used to tell if given IRQ is shared or exclusive. Hence there is a global xarray for that. And my argument is for this case precisely. [other] There is also a separate xarray for each auxdev (IIRC) which is used as generic dynamic container [that stores sysfs attrs], any other would work (with different characteristics), but I see no problems with picking xarray here. > >> The "use least powerful option" rule of thumb is perhaps more important. > > Yes, but use a refcount properly if needed, I can't figure out why a > refcount is needed here at all, which is not a good sign. > >>>> + refcount_set(new_ref, 1); >>>> + ref = __xa_cmpxchg(&irqs, irq, NULL, new_ref, GFP_KERNEL); >>>> + if (ref) { >>>> + kfree(new_ref); >>>> + if (xa_is_err(ref)) { >>>> + ret = xa_err(ref); >>>> + goto out; >>>> + } >>>> + >>>> + /* Another thread beat us to creating the enrtry. */ >>>> + refcount_inc(ref); >>> >>> How can that happen? Why not just use a normal simple lock for all of >>> this so you don't have to mess with refcounts at all? This is not >>> performance-relevent code at all, but yet with a refcount you cause >>> almost the same issues that a normal lock would have, plus the increased >>> complexity of all of the surrounding code (like this, and the crazy >>> __xa_cmpxchg() call) >>> >>> Make this simple please. >> >> I find current API of xarray not ideal for this use case, and would like >> to fix it, but let me write a proper RFC to don't derail (or slow down) >> this series. > > Why do you need to use an xarray here at all? Why isn't this just tied > directly to the aux device instead? for [global] above I find xarray suitable soultion, for the [other] I'll leave defending it to @Shay :) > > thanks, > > greg k-h