linux-kernel - Re: [v3,11/41] mips: reuse asm-generic/barrier.h

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Date:	Thu, 14 Jan 2016 11:28:18 -0800
From:	Leonid Yegoshin <Leonid.Yegoshin@...tec.com>
To:	Will Deacon <will.deacon@....com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	<linux-kernel@...r.kernel.org>, Arnd Bergmann <arnd@...db.de>,
	<linux-arch@...r.kernel.org>,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	<virtualization@...ts.linux-foundation.org>,
	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	Joe Perches <joe@...ches.com>,
	David Miller <davem@...emloft.net>,
	<linux-ia64@...r.kernel.org>, <linuxppc-dev@...ts.ozlabs.org>,
	<linux-s390@...r.kernel.org>, <sparclinux@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>,
	<linux-metag@...r.kernel.org>, <linux-mips@...ux-mips.org>,
	<x86@...nel.org>, <user-mode-linux-devel@...ts.sourceforge.net>,
	<adi-buildroot-devel@...ts.sourceforge.net>,
	<linux-sh@...r.kernel.org>, <linux-xtensa@...ux-xtensa.org>,
	<xen-devel@...ts.xenproject.org>,
	"Ralf Baechle" <ralf@...ux-mips.org>,
	Ingo Molnar <mingo@...nel.org>, <ddaney.cavm@...il.com>,
	<james.hogan@...tec.com>, Michael Ellerman <mpe@...erman.id.au>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Subject: Re: [v3,11/41] mips: reuse asm-generic/barrier.h

On 01/14/2016 04:14 AM, Will Deacon wrote:
> On Wed, Jan 13, 2016 at 02:26:16PM -0800, Leonid Yegoshin wrote:
>
>>      Moreover, there are voices against guarantee that it will be in future
>> and that voices point me to Documentation/memory-barriers.txt section "DATA
>> DEPENDENCY BARRIERS" examples which require SYNC_RMB between loading
>> address/index and using that for loading data based on that address or index
>> for shared data (look on CPU2 pseudo-code):
>>> To deal with this, a data dependency barrier or better must be inserted
>>> between the address load and the data load:
>>>
>>>         CPU 1                 CPU 2
>>>         ===============       ===============
>>>         { A == 1, B == 2, C = 3, P == &A, Q == &C }
>>>         B = 4;
>>>         <write barrier>
>>>         WRITE_ONCE(P, &B);
>>>                               Q = READ_ONCE(P);
>>>                               <data dependency barrier> <-----------
>>> SYNC_RMB is here
>>>                               D = *Q;
>> ...
>>> Another example of where data dependency barriers might be required is
>>> where a
>>> number is read from memory and then used to calculate the index for an
>>> array
>>> access:
>>>
>>>         CPU 1                 CPU 2
>>>         ===============       ===============
>>>         { M[0] == 1, M[1] == 2, M[3] = 3, P == 0, Q == 3 }
>>>         M[1] = 4;
>>>         <write barrier>
>>>         WRITE_ONCE(P, 1);
>>>                               Q = READ_ONCE(P);
>>>                               <data dependency barrier> <------------
>>> SYNC_RMB is here
>>>                               D = M[Q];
>> That voices say that there is a legitimate reason to relax HW here for
>> performance if SYNC_RMB is needed anyway to work with this sequence of
>> shared data.
> Are you saying that MIPS needs to implement [smp_]read_barrier_depends?

It is not me, it is Documentation/memory-barriers.txt from kernel sources.

HW team can't work on voice statements, it should do a work on written 
documents. If that is written (see above the lines which I marked by 
"SYNC_RMB") then anybody should use it and never mind how many 
CPUs/Threads are in play. This examples explicitly requires to insert 
"data dependency barrier" between reading a shared pointer/index and 
using it to fetch a shared data. So, your WRC+addr+addr test is a 
violation of that recommendation.

- Leonid.