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

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Date:	Fri, 6 Feb 2015 14:42:35 -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 10:16:25PM +0000, Mathieu Desnoyers wrote:
> ----- Original Message -----
> > 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>
> > Sent: Friday, February 6, 2015 10:03:59 AM
> > 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?
> 
> The subsystems using llist.h in master: IRQ, smp core code,
> vmalloc, scsi, the block layer, some filesystems... and more.
> grep for "llist.h" to see the complete list of users.
> 
> My question was mainly on how to do the fix process-wise: post-3.18,
> it implies using the new lockless_dereference() API. pre-3.18, we
> need to use smp_read_barrier_depends().
> 
> As Peter Hurley suggested, using the new API for master and 3.18
> seems like the right approach. Then the backports to stable branches
> can, if needed, use smp_read_barrier_depends() instead. Is that
> OK with you ?

Sounds fine to me.

greg k-h
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