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

["Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>]

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?  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.

                                                        Thanx, Paul


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