linux-kernel - Re: [PATCH v2] futex: replace bare barrier() with a READ

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Message-ID: <CAHz2CGX=uh6rvKVyZ21dA95v9wcpBC=8Ldv-3Yjhk6Z3iNtBJw@mail.gmail.com>
Date:	Wed, 2 Mar 2016 21:56:32 +0800
From:	Jianyu Zhan <nasa4836@...il.com>
To:	Christian Borntraeger <borntraeger@...ibm.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>, dave@...olabs.net,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>, linux@...musvillemoes.dk,
	dvhart@...ux.intel.com, bigeasy@...utronix.de,
	schwidefsky@...ibm.com, heiko.carstens@...ibm.com
Subject: Re: [PATCH v2] futex: replace bare barrier() with a READ_ONCE()

On Wed, Mar 2, 2016 at 9:35 PM, Christian Borntraeger
<borntraeger@...ibm.com> wrote:
> That change makes sense. I did the original barrier back in 2006 (could you cc me next time?)

Sorry for that,  I thought get_maintainer.pl would had spit the
original author's email, but apparently it didn't :(

> ACCESS_ONCE or READ_ONCE was not available at that time and its now the better way.
>
> This is not an s390 specific problem, it was just triggered there as the gcc cost model
> considered the memory read as cheap as a register read.
>

Thanks for the clarification.

> Maybe simplify the comment to something like
>
>         /* Prevent the compiler to read lock_ptr twice (if and spin_lock) */

Will do.


Regards,
Jianyu Zhan