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    SubjectRe: [PATCH 1/2] tracing/function-return-tracer: Make the function return tracer lockless
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    Frederic Weisbecker <fweisbec@gmail.com> writes:
    O

    > _ The only race could happen between the current thread and an interrupt. If an
    > interrupt is raised, it will increase the index of the return stack storage and
    > then execute until the end of the tracing to finally free the index it used.
    > We don't need to disable irqs.
    >
    > This is theorical. In practice, I've tested it with a two-core SMP and had no
    > problem at all. Perhaps -tip testing could confirm it.

    The problem I think is that you assume the ++ is atomic against
    interrupts, which is not guaranteed by the C compiler. e.g.
    it would be perfectly legal for the compiler to generate code like

    local register i
    i = index;
    write to index'ed array using i
    <--------- interrupt here would overwrite data
    ...
    index = i + 1;

    You would need to convert the index access to a "local_add_return()" and
    possibly also add memory barriers.

    -Andi

    --
    ak@linux.intel.com


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