linux-kernel - Re: [PATCH v2] barriers: introduce smp_mb__release

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Message-ID: <20151019102324.GO3816@twins.programming.kicks-ass.net>
Date:	Mon, 19 Oct 2015 12:23:24 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Boqun Feng <boqun.feng@...il.com>
Cc:	Will Deacon <will.deacon@....com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	Anton Blanchard <anton@...ba.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v2] barriers: introduce smp_mb__release_acquire and
 update documentation

On Mon, Oct 19, 2015 at 09:17:18AM +0800, Boqun Feng wrote:
> This is confusing me right now. ;-)
> 
> Let's use a simple example for only one primitive, as I understand it,
> if we say a primitive A is "fully ordered", we actually mean:
> 
> 1.	The memory operations preceding(in program order) A can't be
> 	reordered after the memory operations following(in PO) A.
> 
> and
> 
> 2.	The memory operation(s) in A can't be reordered before the
> 	memory operations preceding(in PO) A and after the memory
> 	operations following(in PO) A.
> 
> If we say A is a "full barrier", we actually means:
> 
> 1.	The memory operations preceding(in program order) A can't be
> 	reordered after the memory operations following(in PO) A.
> 
> and
> 
> 2.	The memory ordering guarantee in #1 is visible globally.
> 
> Is that correct? Or "full barrier" is more strong than I understand,
> i.e. there is a third property of "full barrier":
> 
> 3.	The memory operation(s) in A can't be reordered before the
> 	memory operations preceding(in PO) A and after the memory
> 	operations following(in PO) A.
> 
> IOW, is "full barrier" a more strong version of "fully ordered" or not?

Yes, that was how I used it.

Now of course; the big question is do we want to promote this usage or
come up with a different set of words describing this stuff.

I think separating the ordering from the transitivity is useful, for we
can then talk about and specify them independently.

That is, we can say:

	LOAD-ACQUIRE: orders LOAD->{LOAD,STORE}
	              weak transitivity (RCpc)

	MB: orders {LOAD,STORE}->{LOAD,STORE} (fully ordered)
	    strong transitivity (RCsc)

etc..

Also, in the above I used weak and strong transitivity, but that too is
of course up for grabs.
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