On Fri, 2016-11-25 at 17:30 +0100, Lino Sanfilippo wrote: > Hi, > > > > > > The READ_ONCE() are documenting the fact that no lock is taken to fetch > > the stats, while another cpus might being changing them. > > > > I had no answer yet from https://patchwork.ozlabs.org/patch/698449/ > > > > So I thought it was not needed to explain this in the changelog, given > > that it apparently is one of the few things that can block someone to > > understand one of my changes :/ > > > > Apparently nobody really understands READ_ONCE() purpose, it is really a > > pity we have to explain this over and over. > > > > Even at the risk of showing once more a lack of understanding for > READ_ONCE(): > Does not a READ_ONCE() have to e paired with some kind of > WRITE_ONCE()?
You are right. Although in this case, the producers are using a lock, and do ring->packets++; We hopefully have compilers/cpus that do not put intermediate garbage in ring->packets while doing the increment. One problem with : WRITE_ONCE(ring->packets, ring->packets + 1); is that gcc no longer uses an INC instruction. Maybe we need some ADD_ONCE(ptr, val) macro doing the proper thing. > Furthermore: there a quite some network drivers that ensure visibility > of > the descriptor queue indices between xmit and xmit completion function > by means of > smp barriers. Could all these drivers theoretically be adjusted to use > READ_ONCE(), > WRITE_ONCE() for the indices instead? > Well, for this precise case we do need appropriate smp barriers. READ_ONCE() can be better than poor barrier(), look at https://git.kernel.org/cgit/linux/kernel/git/davem/net-next.git/commit/?id=b668534c1d9b80f4cda4d761eb11d3a6c9f4ced8
