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

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Date:	Tue, 20 Oct 2015 15:35:23 +0800
From:	Boqun Feng <boqun.feng@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
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 12:23:24PM +0200, Peter Zijlstra wrote:
> 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.
> 

Great idea! 

> 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)
> 

It will be helpful if we have this kind of description for each
primitive mentioned in memory-barriers.txt, which, IMO, is better than
the description like the following:

"""
Any atomic operation that modifies some state in memory and returns information
about the state (old or new) implies an SMP-conditional general memory barrier
(smp_mb()) on each side of the actual operation (with the exception of
"""

I'm assuming that the arrow "->" stands for the program order, and word
"orders" means that a primitive guarantees some program order becomes
the memory operation order, so that the description above can be
rewritten as:

value-returning atomics:
	orders {LOAD,STORE}->RmW(atomic operation)->{LOAD,STORE}
	strong transitivity
	
much simpler and clearer for discussion and reasoning

Regards,
Boqun

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

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