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    SubjectRe: [PATCH] MTRR: Fix race causing set_mtrr to go into infinite loop
    On 6/25/2007 6:34 PM, Andi Kleen wrote:
    > On Tuesday 26 June 2007 00:05:17 Chuck Ebbert wrote:
    >
    >> On 06/25/2007 05:38 PM, Loic Prylli wrote:
    >>
    >> [cc: Andi]
    >>
    >>
    >>> Processors synchronization in set_mtrr requires the .gate field
    >>> to be set after .count field is properly initialized. Without an explicit
    >>> barrier, the compiler was reordering those memory stores. That was sometimes
    >>> causing a processor (in ipi_handler) to see the .gate change and
    >>> decrement .count before the latter is set by set_mtrr() (which
    >>> then hangs in a infinite loop with irqs disabled).
    >>>
    >
    > Hmm, perhaps we should just put the smp_wmb into atomic_set().
    > Near all other atomic operations have memory barriers too. I think
    > that would be the better fix.
    >
    > -Andi
    >


    In Documentation/atomic_ops.txt atomic_set/atomic_read are described as
    nothing more than a type-safe assignement or reading, without any extra
    semantics. For other atomic operations, the rule is that any atomic
    operation that doesn't return a value doesn't come with a barrier (and
    any operation that returns the atomic value must have memory barriers).

    So I guess you are suggesting to change the doc and the implementation
    for all arches.

    I should admit I did not knew a number of atomic operations did not
    imply memory-barriers before. But maybe the extra cost might not be
    completely negligible, especially if, for consistency with other
    "barrier-implied" atomic operations, a new memory barrier is put before
    and after,

    Are you suggested changing just atomic_set(), or also other barrier-free
    atomic operations :"atomic_dec", "atomic_inc", "atomic_add", "atomic_sub" ?

    Independently of what is done to atomic, what about not making the .gate
    field an atomic_t, but a simple "int" in the mttr code, since the only
    operations done on it are atomic_read and atomic_set?


    Loic

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