On Tue, Jan 12, 2016 at 11:40:12AM +0100, Peter Zijlstra wrote: > On Tue, Jan 12, 2016 at 11:25:55AM +0100, Peter Zijlstra wrote: > > On Tue, Jan 12, 2016 at 10:27:11AM +0100, Peter Zijlstra wrote: > > > 2) the changelog _completely_ fails to explain the sync 0x11 and sync > > > 0x12 semantics nor does it provide a publicly accessible link to > > > documentation that does. > > > > Ralf pointed me at: https://imgtec.com/mips/architectures/mips64/ > > > > > 3) it really should have explained what you did with > > > smp_llsc_mb/smp_mb__before_llsc() in _detail_. > > > > And reading the MIPS64 v6.04 instruction set manual, I think 0x11/0x12 > > are _NOT_ transitive and therefore cannot be used to implement the > > smp_mb__{before,after} stuff. > > > > That is, in MIPS speak, those SYNC types are Ordering Barriers, not > > Completion Barriers. They need not be globally performed. > > Which if true; and I know Will has some questions here; would also mean > that you 'cannot' use the ACQUIRE/RELEASE barriers for your locks as was > recently suggested by David Daney. The issue I have with the SYNC description in the text above is that it describes the single CPU (program order) and the dual-CPU (confusingly named global order) cases, but then doesn't generalise any further. That means we can't sensibly reason about transitivity properties when a third agent is involved. For example, the WRC+sync+addr test: P0: Wx = 1 P1: Rx == 1 SYNC Wy = 1 P2: Ry == 1 <address dep> Rx = 0 I can't find anything to forbid that, given the text. The main problem is having the SYNC on P1 affect the write by P0. > That is, currently all architectures -- with exception of PPC -- have > RCsc locks, but using these non-transitive things will get you RCpc > locks. > > So yes, MIPS can go RCpc for its locks and share the burden of pain with > PPC, but that needs to be a very concious decision. I think it's much worse than RCpc, given my interpretation of the wording. Will -- 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
