On Tue, Sep 15, 2015 at 04:14:39PM +0200, Peter Zijlstra wrote: > On Tue, Sep 15, 2015 at 07:09:22AM -0700, Paul E. McKenney wrote: > > On Tue, Sep 15, 2015 at 02:48:00PM +0200, Peter Zijlstra wrote: > > > On Tue, Sep 15, 2015 at 05:41:42AM -0700, Paul E. McKenney wrote: > > > > > Never mind, the PPC people will implement this with lwsync and that is > > > > > very much not transitive IIRC. > > > > > > > > I am probably lost on context, but... > > > > > > > > It turns out that lwsync is transitive in special cases. One of them > > > > is a series of release-acquire pairs, which can extend indefinitely. > > > > > > > > Does that help in this case? > > > > > > Probably not, but good to know. I still don't think we want to rely on > > > ACQUIRE/RELEASE being transitive in general though. > > > > OK, I will bite... Why not? > > It would mean us reviewing all archs (again) and documenting it I > suppose. Which is of course entirely possible. > > That said, I don't think the case at hand requires it, so lets postpone > this for now ;-)
True enough, but in my experience smp_store_release() and smp_load_acquire() are a -lot- easier to use than other barriers, and transitivity will help promote their use. So... All the TSO architectures (x86, s390, SPARC, HPPA, ...) support transitive smp_store_release()/smp_load_acquire() via their native ordering in combination with barrier() macros. x86 with CONFIG_X86_PPRO_FENCE=y, which is not TSO, uses an mfence instruction. Power supports this via lwsync's partial cumulativity. ARM64 supports it in SMP via the new ldar and stlr instructions (in non-SMP, it uses barrier(), which suffices in that case). IA64 supports this via total ordering of all release instructions in theory and by the actual full-barrier implementation in practice (and the fact that gcc emits st.rel and ld.acq instructions for volatile stores and loads). All other architectures use smp_mb(), which is transitive. Did I miss anything? Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/