On Fri, Aug 28, 2015 at 04:39:21PM +0100, Peter Zijlstra wrote: > On Fri, Aug 28, 2015 at 10:16:02PM +0800, Boqun Feng wrote: > > Ah.. just read through the thread you mentioned, I might misunderstand > > you, probably because I didn't understand RCpc well.. > > > > You are saying that in a RELEASE we -might- switch from smp_lwsync() to > > smp_mb() semantically, right? I guess this means we -might- switch from > > RCpc to RCsc, right? > > > > If so, I think I'd better to wait until we have a conclusion for this. > > Yes, the difference between RCpc and RCsc is in the meaning of RELEASE + > ACQUIRE. With RCsc that implies a full memory barrier, with RCpc it does > not.
We've discussed this before, but for the sake of completeness, I don't think we're fully RCsc either because we don't order the actual RELEASE operation again a subsequent ACQUIRE operation: P0 smp_store_release(&x, 1); foo = smp_load_acquire(&y); P1 smp_store_release(&y, 1); bar = smp_load_acquire(&x); We allow foo == bar == 0, which is prohibited by SC. However, we *do* enforce ordering on any prior or subsequent accesses for the code snippet above (the release and acquire combine to give a full barrier), which makes these primitives well suited to things like message passing. Will _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev
