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

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Date:	Thu, 14 Jan 2016 13:36:50 -0800
From:	Leonid Yegoshin <Leonid.Yegoshin@...tec.com>
To:	<paulmck@...ux.vnet.ibm.com>
CC:	Will Deacon <will.deacon@....com>,
	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>
Subject: Re: [v3,11/41] mips: reuse asm-generic/barrier.h

On 01/14/2016 01:29 PM, Paul E. McKenney wrote:
>
>> On 01/14/2016 12:34 PM, Paul E. McKenney wrote:
>>>
>>> The WRC+addr+addr is OK because data dependencies are not required to be
>>> transitive, in other words, they are not required to flow from one CPU to
>>> another without the help of an explicit memory barrier.
>> I don't see any reliable way to fit WRC+addr+addr into "DATA
>> DEPENDENCY BARRIERS" section recommendation to have data dependency
>> barrier between read of a shared pointer/index and read the shared
>> data based on that pointer. If you have this two reads, it doesn't
>> matter the rest of scenario, you should put the dependency barrier
>> in code anyway. If you don't do it in WRC+addr+addr scenario then
>> after years it can be easily changed to different scenario which
>> fits some of scenario in "DATA DEPENDENCY BARRIERS" section and
>> fails.
> The trick is that lockless_dereference() contains an
> smp_read_barrier_depends():
>
> #define lockless_dereference(p) \
> ({ \
> 	typeof(p) _________p1 = READ_ONCE(p); \
> 	smp_read_barrier_depends(); /* Dependency order vs. p above. */ \
> 	(_________p1); \
> })
>
> Or am I missing your point?

WRC+addr+addr has no any barrier. lockless_dereference() has a barrier. 
I don't see a common points between this and that in your answer, sorry.

- Leonid.