On Thu, Dec 12, 2019 at 05:04:27PM +0000, Will Deacon wrote: > On Thu, Dec 12, 2019 at 11:46:10AM +0100, Peter Zijlstra wrote: > > On Thu, Dec 12, 2019 at 10:07:56AM +0000, Will Deacon wrote: > > > > > > So your proposed change _should_ be fine. Will, I'm assuming you never > > > > saw this on your ARGH64 builds when you did this code ? > > > > > > I did see it, but (a) looking at the code out-of-line makes it look a lot > > > worse than it actually is (so the ext4 example is really helpful -- thanks > > > Michael!) and (b) I chalked it up to a crappy compiler. > > > > > > However, see this comment from Arnd on my READ_ONCE series from the other > > > day: > > > > > > https://lore.kernel.org/lkml/CAK8P3a0f=WvSQSBQ4t0FmEkcFE_mC3oARxaeTviTSkSa-D2qhg@xxxxxxxxxxxxxx > > > > > > In which case, I'm thinking that we should be doing better in READ_ONCE() > > > for non-buggy compilers which would also keep the KCSAN folks happy for this > > > code (and would help with [1] too). > > > > So something like this then? Although I suppose that should be moved > > into compiler-gcc.h and then guarded by #ifndef READ_ONCE or so. > > Ah wait, I think we've been looking at this wrong. The volatile pointer > argument is actually the problem here, not READ_ONCE()! The use of typeof() > means that the temporary variable to which __READ_ONCE_SIZE writes ends up > being a volatile store, so it can't be optimised away. This is why we get > a stack access and why stack protector then wrecks the codegen for us. Hmm, it's actually probably the volatile read which is causing the problem, since __READ_ONCE_SIZE has casted that away and just uses "void *", but you get the idea. Will
