On Thu, Feb 01, 2018 at 02:29:09PM +0100, Peter Zijlstra wrote: > On Thu, Feb 01, 2018 at 09:27:50PM +0900, Stafford Horne wrote: > > I tried to clarify some of this in the spec v1.2 [0] which help formalize > > some of > > the techniques we used for the SMP implementation. Its probably not > > perfect, > > but I added a section "10. Multicore support" and tried to clarify some > > things > > in section 7 on Atomicity. But it seems I dont cover exactly what are are > > mentioning here. In general: > > > > 1 Secondary cores have memory snooping enabled meaning that any write to a > > cached address will cause the cache line to be invalidated. > > 2 l.swa (store atomic word) implies a store buffer flush. > > What about l.lwa? Can that observe 'old' values, or rather, miss values > stuck in a remote store buffer? > > This will then cause the first l.swa to fail, which, per the above, > would then sync things up? Which means you get that one extra > merry-go-round.
That's ok from a correctness perspective, though, as long as store buffers are guaranteed to drain. > > 3 l.msync is used to flush the store buffer > > > > Also, during the IPI controller review [1] Marc Z asked many similar > > questions. > > I believe he was ok in the end. > > > > Anyway, > > Thanks for thanks for spotting the issue here. For some reason I remember > > we > > did have an l.msync for our mb(). Let me think about and test out this > > patch > > (and the fix to actually define mb) to see if anything comes up. > > > > Also, I haven't seen any implementations that use WOM. Stefan might know > > better. > > So if the strong model has a store buffer, as I think the above says, > then it is _NOT_ correct for l.msync to be treated as a NOP, it _must_ > flush the store buffer. > > At which point I think your 'strong' model is basically TSO. So it would > be very good to get that spelled out somewhere. Agreed, especially as a software person reading the spec may end up thinking that they don't need to use l.msync if they never set WOM. At least, I read it this way despite knowing that it's probably not what you intended. Will
