linux-kernel - Re: [Update PATCH 2/2] aio, mem-hotplug: Add memory barrier to aio ring page migration.

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Date:	Tue, 4 Mar 2014 22:04:42 -0500
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	Miao Xie <miaox@...fujitsu.com>
Cc:	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>,
	Tang Chen <tangchen@...fujitsu.com>,
	Benjamin LaHaise <bcrl@...ck.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Jeff Moyer <jmoyer@...hat.com>, guz.fnst@...fujitsu.com,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	linux-aio@...ck.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [Update PATCH 2/2] aio, mem-hotplug: Add memory barrier to aio
 ring page migration.

>> +             /*
>> +              * Ensure that the page's data was copied from old one by
>> +              * aio_migratepage().
>> +              */
>> +             smp_rmb();
>> +
>
> smp_read_barrier_depends() is better.
>
> "One could place an A smp_rmb() primitive between the pointer fetch and
>  dereference. However, this imposes unneeded overhead on systems (such as
>  i386, IA64, PPC, and SPARC) that respect data dependencies on the read side.
>  A smp_read_barrier_depends() primitive has been added to the Linux 2.6 kernel
>  to eliminate overhead on these systems."
>                 -- From Chapter 7.1 of <Memory Barriers: a Hardware View for Software Hackers>
>                    Written by Paul E. McKenney
>

Right.
The document of memory barrier described this situation.


see https://www.kernel.org/doc/Documentation/memory-barriers.txt

CPU 1                            CPU 2
===============      ===============
{ M[0] == 1, M[1] == 2, M[3] = 3, P == 0, Q == 3 }
M[1] = 4;
<write barrier>
ACCESS_ONCE(P) = 1
                                     Q = ACCESS_ONCE(P);
                                     <data dependency barrier>
                                     D = M[Q];
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