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    SubjectRe: Q: smp.c && barriers (Was: [PATCH 1/4] generic-smp: remove single ipi fallback for smp_call_function_many())
    On Tue, Feb 17, 2009 at 02:39:10PM -0800, Paul E. McKenney wrote:
    > On Tue, Feb 17, 2009 at 10:45:18PM +0100, Oleg Nesterov wrote:
    > > > If all of the above is executed by the same task, tripping the BUG_ON()
    > > > means either a compiler or CPU bug.
    > >
    > > I think you misunderstood...
    > >
    > > smp_send_xxx() sends the ipi to another CPU, and smp_xxx_interrupt() is
    > > the handler.
    >
    > You are right, I did miss that completely. :-/
    >
    > I have seen hardware in which the IPI could beat the cache invalidation
    > from the sending CPU to the interrupted CPU, and in which one or both of
    > the CPUs would have to execute special cache-flush/invalidation/whatever
    > instructions for the interrupted CPU to have a consistent view of the
    > data (in your example, "COND").
    >
    > But we had a little chat with the hardware designers, and in subsequent
    > hardware, the IPI interacted with the cache-coherence protocol so as to
    > prevent the above bug from firing. However, this was x86-based hardware,
    > which orders writes. Weakly ordered systems would likely need a memory
    > barrier somewhere, whether as shown above or buried in the smp_send_xxx()
    > primitive.

    I agree with you both that we *should* make arch interrupt code
    do the ordering, but given the subtle lockups on some architectures
    in this new code, I didn't want to make it significantly weaker...

    Though perhaps it appears that I have, if I have removed an smp_mb
    that x86 was relying on to emit an mfence to serialise the apic.


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