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    SubjectRe: [PATCH 1/3] perf_event: fix race in perf_swevent_get_recursion_context()


    Frederic Weisbecker wrote:
    > On Mon, Jan 18, 2010 at 09:42:34PM +0800, Xiao Guangrong wrote:
    >> It only disable preemption in perf_swevent_get_recursion_context()
    >> it can't avoid race of hard-irq and NMI
    >>
    >> In this patch, we use atomic operation to avoid it and reduce
    >> cpu_ctx->recursion size, it also make this patch no need diable
    >> preemption
    >>
    >> Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
    >
    >
    >
    > I don't understand what is racy in what we have currently.
    >

    It's because hard-irq(we can handle interruption with interruption enabled)
    and NMI are nested, for example:

    int perf_swevent_get_recursion_context(void)
    {
    ......
    if (cpuctx->recursion[rctx]) {
    put_cpu_var(perf_cpu_context);
    return -1;
    }
    /*
    * Another interruption handler/NMI will re-enter there if it
    * happed, it make the recursion value chaotic
    */
    cpuctx->recursion[rctx]++;
    ......
    }
    >
    >
    >> int perf_swevent_get_recursion_context(void)
    >> {
    >> - struct perf_cpu_context *cpuctx = &get_cpu_var(perf_cpu_context);
    >> + struct perf_cpu_context *cpuctx = &__get_cpu_var(perf_cpu_context);
    >> int rctx;
    >>
    >> if (in_nmi())
    >> @@ -3933,13 +3933,8 @@ int perf_swevent_get_recursion_context(void)
    >> else
    >> rctx = 0;
    >>
    >> - if (cpuctx->recursion[rctx]) {
    >> - put_cpu_var(perf_cpu_context);
    >> + if (test_and_set_bit(rctx, &cpuctx->recursion))
    >> return -1;
    >
    >
    >
    > This looks broken. We don't call back perf_swevent_put_recursion_context
    > in fail case, so the bit won't ever be cleared once we recurse.
    >

    Um, i think we can't clear the bit in this fail case, consider below
    sequence:

    path A: path B

    set bit but find the bit already set
    atomic set bit |
    | |
    V |
    handle SW event |
    | V
    V exit and not clear the bit
    atomic clear bit
    After A and B, the bit is still zero

    Right? :-)

    Thanks,
    Xiao


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