linux-kernel - Re: [PATCH] llist: Fix missing memory barrier

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Date:	Fri, 6 Feb 2015 07:03:59 -0800
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	Pranith Kumar <bobby.prani@...il.com>,
	Huang Ying <ying.huang@...el.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	David Howells <dhowells@...hat.com>
Subject: Re: [PATCH] llist: Fix missing memory barrier

On Fri, Feb 06, 2015 at 02:12:32PM +0000, Mathieu Desnoyers wrote:
> ----- Original Message -----
> > From: "Pranith Kumar" <bobby.prani@...il.com>
> > To: "Mathieu Desnoyers" <mathieu.desnoyers@...icios.com>
> > Cc: "Huang Ying" <ying.huang@...el.com>, "LKML" <linux-kernel@...r.kernel.org>, "Paul McKenney"
> > <paulmck@...ux.vnet.ibm.com>, "David Howells" <dhowells@...hat.com>
> > Sent: Thursday, February 5, 2015 10:44:07 PM
> > Subject: Re: [PATCH] llist: Fix missing memory barrier
> > 
> > Hi Mathieu,
> > 
> > On Thu, Feb 5, 2015 at 10:06 PM, Mathieu Desnoyers
> > <mathieu.desnoyers@...icios.com> wrote:
> > > A smp_read_barrier_depends() appears to be missing in llist_del_first().
> > > It should only matter for Alpha in practice. Adding it after the check
> > > of entry against NULL allows skipping the barrier in a common case.
> > 
> > We recently decided on using lockless_dereference() instead of
> > hard-coding smp_read_barrier_depends()[1]. The advantage is that
> > lockless_dereference() clearly shows what loads are being ordered.
> > Could you resend the patch using that API?
> 
> Since llist.h has been introduced prior to 3.18, I'm wondering if
> it would be worthwhile to submit 2 patches for the purpose of
> backporting to stable branches:
> 
> 1) Fix introducing smp_read_barrier_depends() (for master and
>    stable branches)
> 2) Move master from smp_read_barrier_depends() to
>    lockless_dereference(),
> 
> Thoughts ?

Yes, why?  What code needs these new apis?

confused,

greg k-h
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