Re: [PATCH -tip 00/12] locking/atomics: Add and use inc,dec calls for FETCH-OP flavors

[Davidlohr Bueso]

On Fri, 24 Jun 2016, James Bottomley wrote:

> On Mon, 2016-06-20 at 13:05 -0700, Davidlohr Bueso wrote:
> > Hi,
> >
> > The series is really straightforward and based on Peter's work that
> > introduces[1] the atomic_fetch_$op machinery. Only patch 1 implements
> > the actual atomic_fetch_{inc,dec} calls based on
> > atomic_fetch_{add,sub}.
>
> Could I just ask why?  atomic_inc_return(x) - 1 seems a reasonable
> thing to do to me.

For one restoring the old state like that can be racy and looses the notion
of atomicity. The new family of atomic_fetch_$ops also better express the
purpose of the call imo. Finally, the added machinery (considering it came
from fetch_op() NOHZ needs), was mainly suggested by Linus (although yes, we
don't have users for all the calls): https://lkml.org/lkml/2016/3/15/352.

Thanks,
Davidlohr