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

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Message-ID: <3908561D78D1C84285E8C5FCA982C28F31CDCCFA@ORSMSX106.amr.corp.intel.com>
Date:	Thu, 5 Sep 2013 19:45:16 +0000
From:	"Luck, Tony" <tony.luck@...el.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Heiko Carstens <heiko.carstens@...ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] lockref: remove cpu_relax() again

>> 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..

No.  I can change the Linux code to say "cmpxchg.rel" here ... but the
h/w will do exactly the same thing it did when I had "cmpxchg.acq".

-Tony