netdev - Re: [PATCH RFC 0/4] barriers using data dependency

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Date:   Wed, 2 Jan 2019 16:36:40 -0500 (EST)
From:   Alan Stern <stern@...land.harvard.edu>
To:     "Michael S. Tsirkin" <mst@...hat.com>
cc:     linux-kernel@...r.kernel.org, Jason Wang <jasowang@...hat.com>,
        Andrea Parri <andrea.parri@...rulasolutions.com>,
        Will Deacon <will.deacon@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Akira Yokosawa <akiyks@...il.com>,
        Daniel Lustig <dlustig@...dia.com>,
        <linux-arch@...r.kernel.org>, <netdev@...r.kernel.org>,
        <virtualization@...ts.linux-foundation.org>
Subject: Re: [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(-)