On Mon, 3 Sep 2018, Andrea Parri wrote: > I take this opportunity to summarize my viewpoint on these matters: > > Someone would have to write the commit message for the above diff ... > that is, to describe -why- we should go RCtso (and update the documen- > tation accordingly); by now, the only argument for this appears to be: > "(most) people expect strong ordering" _and they will be "lazy enough" > to not check their expectations by using the LKMM tool (paraphrasing > from [1]); IAC, Linux "might work" better if we add this ordering to > the LKMM. Agreeing on such an approach would mean agreeing that this > argument "wins" over: > > "We want new architectures to implement acquire/release efficiently, > and it's not unlikely that they will have acquire loads that are > similar in semantics to LDAPR." [2] > > "RISC-V probably would have been RCpc [...] it takes extra fences > to go from RCpc to either "RCtso" or RCsc." [3] > > (or similar instances) since, of course, there is no such thing as a > "free strong ordering"; and I'm not only talking about "efficiency", > I'm also thinking at the fact that someone will have to maintain that > ordering across all the architectures and in the LKMM. > > If, OTOH, we agree that the above "win"/assumption is valid only for > locks or, in other/better words, if we agree that we should maintain > _two_ distinct release-acquire orderings (a first one for unlock-lock > sequences and a second one for ordinary/atomic release-acquire, say, > as proposed in the patch under RFC), In fact, there have have been _two_ proposals along this line. One as you describe here (which is what the 1/7 patch under discussion does), and another in which unlock-lock sequences and atomic acquire-release sequences both have "RCtso" semantics while ordinary acquire/release sequences have RCpc semantics. You should consider the second proposal. It could be put into the LKMM quite easily by building upon this 1/7 patch. > I ask that we audit and modify > the generic code accordingly/as suggested in other posts _before_ we > upstream the changes for the LKMM: we should identify those places > where (the newly introduced) _gap_ between unlock-lock and the other > release-acquire is not admissible and fix those places (notice that > this entails, in part., agreeing on what/where the generic code is). Have you noticed any part of the generic code that relies on ordinary acquire-release (rather than atomic RMW acquire-release) in order to implement locking constructs? > Finally, if we don't agree with the above assumption at all (that is, > no matter if we are considering unlock-lock or other release-acquire > sequences), then we should go RCpc [4]. > > I described three different approaches (which are NOT "independent", > clearly; let us find an agreement...); even though some of them look > insane to me, I'm currently open to all of them: thoughts? How about this fourth approach? Alan > Andrea > > [1] http://lkml.kernel.org/r/20180712134821.GT2494@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx > http://lkml.kernel.org/r/CA+55aFwKpkU5C23OYt1HCiD3X5bJHVh1jz5G2dSnF1+kVrOCTA@xxxxxxxxxxxxxx > [2] http://lkml.kernel.org/r/20180622183007.GD1802@xxxxxxx > [3] http://lkml.kernel.org/r/11b27d32-4a8a-3f84-0f25-723095ef1076@xxxxxxxxxx > [4] http://lkml.kernel.org/r/20180711123421.GA9673@andrea > http://lkml.kernel.org/r/Pine.LNX.4.44L0.1807132133330.26947-100000@xxxxxxxxxxxxxxxxxxxx
