On Fri, May 22, 2020 at 11:44:07AM +0200, Peter Zijlstra wrote: > On Thu, May 21, 2020 at 05:38:50PM -0700, Paul E. McKenney wrote: > > Hello! > > > > Just wanted to call your attention to some pretty cool and pretty serious > > litmus tests that Andrii did as part of his BPF ring-buffer work: > > > > https://lore.kernel.org/bpf/20200517195727.279322-3-andriin@xxxxxx/ > > > > Thoughts? > > I find: > > smp_wmb() > smp_store_release() > > a _very_ weird construct. What is that supposed to even do? Indeed, and I asked about that in my review of the patch containing the code. It -could- make sense if there is a prior read and a later store: r1 = READ_ONCE(a); WRITE_ONCE(b, 1); smp_wmb(); smp_store_release(&c, 1); WRITE_ONCE(d, 1); So a->c and b->c is smp_store_release() and b->d is smp_wmb(). But if there were only stores, the smp_wmb() would suffice. And if there wasn't the trailing store, smp_store_release() would suffice. But that would at least want a comment, in my opinion. ;-) Thanx, Paul
