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    SubjectRe: [PATCH] oprofile: fix CPU unplug panic in ppro_stop()
    Robert Richter wrote:
    > On 02.12.08 09:17:29, Ingo Molnar wrote:
    >> * Eric Dumazet <dada1@cosmosbay.com> wrote:
    >>
    >>> If oprofile statically compiled in kernel, a cpu unplug triggers
    >>> a panic in ppro_stop(), because a NULL pointer is dereferenced.
    >>>
    >>> Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
    >>> ---
    >>> arch/x86/oprofile/op_model_ppro.c | 4 ++++
    >>> 1 files changed, 4 insertions(+)
    >>> diff --git a/arch/x86/oprofile/op_model_ppro.c b/arch/x86/oprofile/op_model_ppro.c
    >>> index 716d26f..e9f80c7 100644
    >>> --- a/arch/x86/oprofile/op_model_ppro.c
    >>> +++ b/arch/x86/oprofile/op_model_ppro.c
    >>> @@ -156,6 +156,8 @@ static void ppro_start(struct op_msrs const * const msrs)
    >>> unsigned int low, high;
    >>> int i;
    >>>
    >>> + if (!reset_value)
    >>> + return;
    >>>
    >>> for (i = 0; i < num_counters; ++i) {
    >>> if (reset_value[i]) {
    >>> CTRL_READ(low, high, msrs, i);
    >
    > The patch fixes the null pointer access and this ok. But the root
    > cause seems to be in the cpu hotplug and initialization
    > code. xxx_start() should not be called before xxx_setup_ctrs() or
    > after xxx_shutdown().

    Yes, it would be better to fix that. At least it would make
    the code cleaner than the add checks for this backdoor everywhere.

    > Also, running only xxx_start() and xxx_stop() in
    > the cpu notifier functions is not sufficient. There is at least some
    > on_each_cpu code in nmi_setup() that should be called also in the cpu
    > notifier functions. I have to review that code.

    AFAIK cpu hotplug has more problems in oprofile anyways. That is why
    I didn't test that case.

    >
    > [...]
    >
    >> It was absolutely unnecessary to add kmalloc to this rarely executed
    >> codepath - and the way it was added was absolutely horrible as well, it
    >> was tacked on in the middle of an existing codepath, instead of factoring
    >> it out nicely. Perfmon will eventually replace PMC management anyway, so
    >> there was no "this way it's cleaner" argument either. So this code should
    >> have been changed minimally, instead of slapping in a full kmalloc for a
    >> simple array extension from 2 to 4 entries ...
    >
    > Ingo, you are right that using kmalloc is unnecessary for
    > reset_value. So, Andi, maybe you could make this code easier?

    The reason I added the kmalloc is that there's also a varying number
    of separate fixed function counters (although that's not currently
    submitted).

    Also I would prefer to not have a hard coded number for future
    CPUs. Contrary to other people's opinion architectural perfmon is
    not for Nehalem only.

    -Andi


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