On Wed, Jan 02, 2019 at 04:36:40PM -0500, Alan Stern wrote: > On Wed, 2 Jan 2019, Michael S. Tsirkin wrote: > > > So as explained in Documentation/memory-barriers.txt e.g. > > a load followed by a store require a full memory barrier, > > to avoid store being ordered before the load. > > Similarly load-load requires a read memory barrier. > > > > Thinking about it, we can actually create a data dependency > > by mixing the first loaded value into the pointer being > > accessed. > > > > This adds an API for this and uses it in virtio. > > > > Written over the holiday and build tested only so far. > > You are using the terminology from memory-barriers.txt, referring to > the new dependency you create as a data dependency. However, > tools/memory-model/* uses a more precise name, calling it an address > dependency. Could you change the comments in the patches to use this > name instead? Sure, sounds good. While I'm at it, should memory-barriers.txt be switched over too? > > This patchset is also suboptimal on e.g. x86 where e.g. smp_rmb is a nop. > > This should be easy to fix with an architecture-specific override. > > Alan Stern Absolutely. It does however mean that we'll need several variants: mb/rmb, smp/dma/virt/mandatory. I am still trying to decide whether it's good since it documents the kind of barrier that we are trying to use - or bad since it's more verbose and makes you choose one where they are all pretty cheap. > > Sending out for early feedback/flames. > > > > Michael S. Tsirkin (4): > > include/linux/compiler*.h: fix OPTIMIZER_HIDE_VAR > > include/linux/compiler.h: allow memory operands > > barriers: convert a control to a data dependency > > virtio: use dependent_ptr_mb > > > > Documentation/memory-barriers.txt | 20 ++++++++++++++++++++ > > arch/alpha/include/asm/barrier.h | 1 + > > drivers/virtio/virtio_ring.c | 6 ++++-- > > include/asm-generic/barrier.h | 18 ++++++++++++++++++ > > include/linux/compiler-clang.h | 5 ++--- > > include/linux/compiler-gcc.h | 4 ---- > > include/linux/compiler-intel.h | 4 +--- > > include/linux/compiler.h | 8 +++++++- > > 8 files changed, 53 insertions(+), 13 deletions(-)
