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Re: [Xen-devel] [v3,11/41] mips: reuse asm-generic/barrier.h


On Thu, Jan 14, 2016 at 02:20:46PM -0800, Paul E. McKenney wrote:
> On Thu, Jan 14, 2016 at 01:24:34PM -0800, Leonid Yegoshin wrote:
> > It is not so simple, I mean "local ordering for address and data
> > dependencies". Local ordering is NOT enough. It happens that current
> > MIPS R6 doesn't require in your example smp_read_barrier_depends()
> > but in discussion it comes out that it may not. Because without
> > smp_read_barrier_depends() your example can be a part of Will's
> > WRC+addr+addr and we found some design which easily can bump into
> > this test. And that design actually performs "local ordering for
> > address and data dependencies" too.
> As noted in another email in this thread, I do not believe that
> WRC+addr+addr needs to be prohibited.  Sounds like Will and I need to
> get our story straight, though.

I think you figured this out while I was sleeping, but just to confirm:

 1. The MIPS64 ISA doc [1] talks about SYNC in a way that applies only
    to memory accesses appearing in *program-order* before the SYNC

 2. We need WRC+sync+addr to work, which means that the SYNC in P1 must
    also capture the store in P0 as being "before" the barrier. Leonid
    reckons it works, but his explanation [2] focussed on the address
    dependency in P2 as to why this works. If that is the case (i.e.
    address dependency provides global transitivity), then WRC+addr+addr
    should also work (even though its not required).

 3. It seems that WRC+addr+addr doesn't work, so I'm still suspicious
    about WRC+sync+addr, because neither the architecture document or
    Leonid's explanation tell me that it should be forbidden.


[1] https://imgtec.com/?do-download=4302
[2] http://lkml.kernel.org/r/569565DA.2010903@xxxxxxxxxx (scroll to the end)

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