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SubjectRe: [PATCH RFC 0/4] barriers using data dependency
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(-)

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