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    SubjectRe: Random panic in load_balance() with 3.16-rc
    On Wed, Jul 23, 2014 at 09:54:23AM -0700, Linus Torvalds wrote:
    > On Wed, Jul 23, 2014 at 8:55 AM, Peter Zijlstra <peterz@infradead.org> wrote:
    > >>
    > >> I haven't seen the full oops, can you forward the screenshot? The
    > >> exact register state might give some clues.
    > >
    > > Sure, here goes.
    >
    > So the length is fine, and the disassembly shows that it is fixed (16
    > 32-bit words - why the heck does it use "movsl" rather than "movsq",
    > whatever).
    >
    > The problem is %rdi, which has the value ffff10043c803e8c, which isn't
    > canonical. Which is why it GP-faults.
    >
    > That value is loaded from the stack:
    >
    > mov -0x88(%rbp),%rdi
    >
    > so apparently the original "__get_cpu_var(load_balance_mask)" is
    > already corrupted, or something has corrupted it on the stack since
    > loading (but that looks unlikely).
    >
    > And I wonder if I have a clue. Look, load_balance_mask is a
    > "cpumask_var_t", but I don't see a "alloc_cpumask_var()" for it.
    > That's broken with CONFIG_CPUMASK_OFFSTACK.

    kernel/sched/core.c:sched_init()

    plays horrible allocation tricks.. which I suppose we should clean up,
    sched_init() appears to be called late enough to use regular per-cpu
    allocations.

    > I think you actually want "load_balance_mask" to be a "struct cpumask *", no?
    >
    > Alternatively, keep it a "cpumask_var_t", but then you need to use
    > __get_cpu_pointer() to get the address of it, and use
    > "alloc_cpumask_var()" to allocate area for the OFFSTACK case.

    I'm always terminally confused on that interface.. but this code hasn't
    changed in a long while and I would expect other crashes if this was
    really funky like that.


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