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

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Date:	Thu, 14 Jan 2016 13:45:44 -0800
From:	Leonid Yegoshin <Leonid.Yegoshin@...tec.com>
To:	<paulmck@...ux.vnet.ibm.com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Will Deacon <will.deacon@....com>,
	"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 01:34 PM, Paul E. McKenney wrote:
> On Thu, Jan 14, 2016 at 12:46:43PM -0800, Leonid Yegoshin wrote:
>> On 01/14/2016 12:15 PM, Peter Zijlstra wrote:
>>> On Thu, Jan 14, 2016 at 11:42:02AM -0800, Leonid Yegoshin wrote:
>>>> An the only point - please use an appropriate SYNC_* barriers instead of
>>>> heavy bold hammer. That stuff was design explicitly to support the
>>>> requirements of Documentation/memory-barriers.txt
>>> That's madness. That document changes from version to version as to what
>>> we _think_ the actual hardware does. It is _NOT_ a specification.
>>>
>>> You cannot design hardware from that. Its incomplete and fails to
>>> specify a bunch of things. It not a mathematically sound definition of a
>>> memory model.
>>>
>>> Please stop referring to that document for what a particular barrier
>>> _should_ do.  Explain what MIPS does, so we can attempt to integrate
>>> this knowledge with our knowledge of PPC/ARM/Alpha/x86/etc. and improve
>>> upon our understanding of hardware and improve the Linux memory model.
>> I am afraid I can't help you here. It is very complicated stuff and
>> a model is actually doesn't fit your assumptions about CPUs well
>> without some simplifications which are based on what you want to
>> have.
>>
>> I say that SYNC_ACQUIRE/etc follows what you expect for smp_acquire
>> etc (basing on that document). And at least two CPU models were
>> tested with my patches (see it in LMO) for that last year and that
>> instructions are implemented now in engineering kernel.
>>
>> If you have something else in mind, you can ask me. But I prefer to
>> do not deviate too much from Documentation/memory-barriers.txt, for
>> exam - if it asks to have memory barrier somewhere, then I assume
>> the code should have it, and please - don't ask me a test which
>> violates the current version of document recommendations.
>>
>> For a moment I don't see a significant changes in this document for
>> MIPS Arch at least 1.5 year, and the only significant point is that
>> MIPS CPU Arch doesn't have yet smp_read_barrier_depends() and
>> smp_rmb() should be used instead.
> Is SYNC_ACQUIRE a memory-barrier instruction that orders prior loads
> against later loads and stores?

Yes, it is in MD00087 (table 6.6 of document Ver 6.04) - 
https://imgtec.com/?do-download=4302

>    If so, and if MIPS does not do
> ordering based on address and data dependencies, I suggest making
> read_barrier_depends() be a SYNC_ACQUIRE rather than SYNC_RMB.

I understood that, after I see the example of using it.
Please consider to add that into Documentation/memory-barriers.txt (it 
is not easy to find that this barrier is used for shared WRITE basing on 
shared pointer), it would be helpful.

- Leonid.