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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