On Fri, Feb 07, 2014 at 12:01:25PM +0000, Will Deacon wrote: > Hello Torvald, > > It looks like Paul clarified most of the points I was trying to make > (thanks Paul!), so I won't go back over them here. > > On Thu, Feb 06, 2014 at 09:09:25PM +0000, Torvald Riegel wrote: > > Are you familiar with the formalization of the C11/C++11 model by Batty > > et al.? > > http://www.cl.cam.ac.uk/~mjb220/popl085ap-sewell.pdf > > http://www.cl.cam.ac.uk/~mjb220/n3132.pdf > > > > They also have a nice tool that can run condensed examples and show you > > all allowed (and forbidden) executions (it runs in the browser, so is > > slow for larger examples), including nice annotated graphs for those: > > http://svr-pes20-cppmem.cl.cam.ac.uk/cppmem/ > > Thanks for the link, that's incredibly helpful. I've used ppcmem and armmem > in the past, but I didn't realise they have a version for C++11 too. > Actually, the armmem backend doesn't implement our atomic instructions or > the acquire/release accessors, so it's not been as useful as it could be. > I should probably try to learn OCaml... That would be very cool! Another option would be to recruit a grad student to take on that project for Peter Sewell. He might already have one, for all I know. > > IMHO, one thing worth considering is that for C/C++, the C11/C++11 is > > the only memory model that has widespread support. So, even though it's > > a fairly weak memory model (unless you go for the "only seq-cst" > > beginners advice) and thus comes with a higher complexity, this model is > > what likely most people will be familiar with over time. Deviating from > > the "standard" model can have valid reasons, but it also has a cost in > > that new contributors are more likely to be familiar with the "standard" > > model. > > Indeed, I wasn't trying to write-off the C11 memory model as something we > can never use in the kernel. I just don't think the current situation is > anywhere close to usable for a project such as Linux. If a greater > understanding of the memory model does eventually manifest amongst C/C++ > developers (by which I mean, the beginners advice is really treated as > such and there is a widespread intuition about ordering guarantees, as > opposed to the need to use formal tools), then surely the tools and libraries > will stabilise and provide uniform semantics across the 25+ architectures > that Linux currently supports. If *that* happens, this discussion is certainly > worth having again. And it is likely to be worthwhile even before then on a piecemeal basis, where architecture maintainers pick and choose which primitive is in inline assembly and which the compiler can deal with properly. For example, I bet that atomic_inc() can be implemented just fine by C11 in the very near future. However, atomic_add_return() is another story. Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
