linux-kernel - Re: [PATCH] lockref: remove cpu

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Message-ID: <CA+55aFw_Rovv7YTb62q5NQvucM5ks8+HXcsTOJ-oJ+kqmHop4A@mail.gmail.com>
Date:	Thu, 5 Sep 2013 12:21:10 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"Luck, Tony" <tony.luck@...el.com>
Cc:	Heiko Carstens <heiko.carstens@...ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] lockref: remove cpu_relax() again

On Thu, Sep 5, 2013 at 11:57 AM, Luck, Tony <tony.luck@...el.com> wrote:
>
> Not "tons", just two.  You can ask for "acquire" or "release" semantics,
> there is no relaxed option.

Seriously? You can't just do a cache-coherent cmpxchg without extra
serialization? Oh well.

> Worse still - early processor implementations actually just ignored
> the acquire/release and did a full fence all the time.  Unfortunately
> this meant a lot of badly written code that used .acq when they really
> wanted .rel became legacy out in the wild - so when we made a cpu
> that strictly did the .acq or .rel ... all that code started breaking - so
> we had to back-pedal and keep the "legacy" behavior of a full fence :-(

Ugh. Can you try what happens with the weaker release-semantics
performance-wise for that code? Do it *just* for the lockref code..

         Linus
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