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

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Date:	Wed, 13 Jan 2016 11:02:35 -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 01/13/2016 02:45 AM, Will Deacon wrote:
> On Tue, Jan 12, 2016 at 12:45:14PM -0800, Leonid Yegoshin wrote:
>>
> I don't think the address dependency is enough on its own. By that
> reasoning, the following variant (WRC+addr+addr) would work too:
>
>
> P0:
> Wx = 1
>
> P1:
> Rx == 1
> <address dep>
> Wy = 1
>
> P2:
> Ry == 1
> <address dep>
> Rx = 0
>
>
> So are you saying that this is also forbidden?
> Imagine that P0 and P1 are two threads that share a store buffer. What
> then?
>

I ask HW team about it but I have a question - has it any relationship 
with replacing MIPS SYNC with lightweight SYNCs (SYNC_WMB etc)? You use 
any barrier or do not use it and I just voice an intention to use a more 
efficient instruction instead of bold hummer (SYNC instruction). If you 
don't use any barrier here then it is a different issue.

May be it has sense to return back to original issue?

- Leonid