Messages in this thread | | | Date | Mon, 14 Jul 2014 13:57:38 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH tip/core/rcu 3/4] documentation: Add acquire/release barriers to pairing rules |
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On Tue, Jul 08, 2014 at 08:31:17AM -0700, Paul E. McKenney wrote:
> Good point, how about the following? > > General barriers pair with each other, though they also pair > with most other types of barriers, albeit without transitivity.
> An acquire barrier pairs with a release barrier, but both may also > pair with other barriers, including of course general barriers.
> A write barrier pairs with a data dependency barrier, an acquire > barrier, a release barrier, a read barrier, or a general barrier.
> Similarly a read barrier or a data dependency barrier pairs > with a write barrier, an acquire barrier, a release barrier, > or a general barrier:
It might be clearer with the added whitespace, or as an explicit list I suppose, but yes.
> > Also, it might be good to have a section on the ramifications of pairing > > acquire/release with other than themselves, I have the feeling there's > > subtle things there. > > It can get quite subtle. For the time being, I am dodging this subtlety > by saying that only general barriers provide transitivity (see the > "TRANSITIVITY" section).
Ah, I was more thinking of the fact that ACQUIRE/RELEASE are semi-permeable while READ/WRITE are memop dependent.
So any combination will be a semi-permeable memop dependent thing, which is the most narrow barrier possible.
So if we thing of ACQUIRE/RELEASE as being 'half' a full barrier, separated in direction, and READ/WRITE as being 'half' a full barrier separated on type, then the combination is a 'quarter' barrier.
Not arguing they're not useful, just saying we need to be extra careful.
> Maybe some day we should capture this subtlety in memory-barriers.txt, > but we will first need a new generation of small children who are not > scared by the current document. ;-)
Lolz :-) [unhandled content-type:application/pgp-signature] | |