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?

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

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(-)