Hi Eric, On 25.11.2016 20:19, Eric Dumazet wrote: > 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.
I see. So we would have to do something like tmp = ring->packets; tmp++; WRITE_ONCE(ring->packets, tmp); to use WRITE_ONCE in this case? If so, could it be worth doing something like this to have a balanced READ_ONCE, WRITE_ONCE usage? > > 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 > > Regards, Lino
