Messages in this thread | | | Date | Mon, 19 Oct 2015 12:23:24 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v2] barriers: introduce smp_mb__release_acquire and update documentation |
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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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