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From: Noah Goldstein <goldstein.w.n@gmail.com>
Date: Tue, 12 Jul 2022 12:18:50 -0700
Message-ID: <CAFUsyfJfYGGczz_G9RWnoo9rPpnkfiQBAh5bYJy_pfDotdPMLw@mail.gmail.com>
Subject: Re: [PATCH 3/7] Remove atomic_bit_set/bit_test_set
To: "H.J. Lu" <hjl.tools@gmail.com>
Cc: Wilco Dijkstra <Wilco.Dijkstra@arm.com>, 
 Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>,
 GNU C Library <libc-alpha@sourceware.org>
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On Tue, Jul 12, 2022 at 12:11 PM H.J. Lu <hjl.tools@gmail.com> wrote:
>
> On Tue, Jul 12, 2022 at 11:44 AM Wilco Dijkstra <Wilco.Dijkstra@arm.com> wrote:
> >
> > Hi,
> >
> > I don't believe it could make a difference since it isn't performance critical code.
> > Note that several similar fetch_or/fetch_and are used which do not optimize into a
>
> Only the operations with test have this issue.  Simple fetch_or and
> fetch_and are
> OK.
>
> > bitset operation, so even if we fix this particular case to use unsigned, there are many
> > others. IIRC there is even a fetch_or with zero which is kind of useless!
> >
> > More importantly, it raises a question for locking: we currently use a compare-exchange
> > for each lock and unlock. If compare-exchange has higher overheads than other atomics
> > then we should investigate more efficient locking sequences.
>
> compare-exchange can be very expensive under contention.
>

Which locks should we look into dropping CAS? pthread_mutex? Anything else?
> > Cheers,
> > Wilco
>
>
>
> --
> H.J.