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

[Leonid Yegoshin <Leonid.Yegoshin@xxxxxxxxxx>]

On 01/14/2016 01:34 PM, Paul E. McKenney wrote:
> On Thu, Jan 14, 2016 at 12:46:43PM -0800, Leonid Yegoshin wrote:
> > On 01/14/2016 12:15 PM, Peter Zijlstra wrote:
> > > On Thu, Jan 14, 2016 at 11:42:02AM -0800, Leonid Yegoshin wrote:
> > > > An the only point - please use an appropriate SYNC_* barriers instead of
> > > > heavy bold hammer. That stuff was design explicitly to support the
> > > > requirements of Documentation/memory-barriers.txt
> > > That's madness. That document changes from version to version as to what
> > > we _think_ the actual hardware does. It is _NOT_ a specification.
> > >
> > > You cannot design hardware from that. Its incomplete and fails to
> > > specify a bunch of things. It not a mathematically sound definition of a
> > > memory model.
> > >
> > > Please stop referring to that document for what a particular barrier
> > > _should_ do.  Explain what MIPS does, so we can attempt to integrate
> > > this knowledge with our knowledge of PPC/ARM/Alpha/x86/etc. and improve
> > > upon our understanding of hardware and improve the Linux memory model.
> > I am afraid I can't help you here. It is very complicated stuff and
> > a model is actually doesn't fit your assumptions about CPUs well
> > without some simplifications which are based on what you want to
> > have.
> >
> > I say that SYNC_ACQUIRE/etc follows what you expect for smp_acquire
> > etc (basing on that document). And at least two CPU models were
> > tested with my patches (see it in LMO) for that last year and that
> > instructions are implemented now in engineering kernel.
> >
> > If you have something else in mind, you can ask me. But I prefer to
> > do not deviate too much from Documentation/memory-barriers.txt, for
> > exam - if it asks to have memory barrier somewhere, then I assume
> > the code should have it, and please - don't ask me a test which
> > violates the current version of document recommendations.
> >
> > For a moment I don't see a significant changes in this document for
> > MIPS Arch at least 1.5 year, and the only significant point is that
> > MIPS CPU Arch doesn't have yet smp_read_barrier_depends() and
> > smp_rmb() should be used instead.
> Is SYNC_ACQUIRE a memory-barrier instruction that orders prior loads
> against later loads and stores?
Yes, it is in MD00087 (table 6.6 of document Ver 6.04) - 
https://imgtec.com/?do-download=4302
>    If so, and if MIPS does not do
> ordering based on address and data dependencies, I suggest making
> read_barrier_depends() be a SYNC_ACQUIRE rather than SYNC_RMB.
I understood that, after I see the example of using it.
Please consider to add that into Documentation/memory-barriers.txt (it 
is not easy to find that this barrier is used for shared WRITE basing on 
shared pointer), it would be helpful.
- Leonid.



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