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    SubjectRe: lockdep warning in check_flags()

    * Frederik Deweerdt <deweerdt@free.fr> wrote:

    > Lockdep issues the following warning:
    >
    > [ 16.835268] Freeing unused kernel memory: 260k freed
    > [ 16.842715] Write protecting the kernel read-only data: 432k
    > [ 17.796518] BUG: warning at kernel/lockdep.c:2359/check_flags()

    this warning means that the "soft" and "hard" hardirqs-disabled state
    got out of sync: the irqtrace tracking code thinks that hardirqs are
    disabled, while in reality they are enabled. The thing to watch for are
    new "stii" instructions in entry.S (and other assembly code), without a
    matching TRACE_HARDIRQS_ON call. [Another, rarer possiblity is NMI code
    saving/restoring interrupts - do you have NMIs enabled? (are there any
    NMI counts in /proc/interrupts?)]

    lockdep automatically generates a minimal trace of hardirqs-off
    state-setting:

    > [ 17.885839] irq event stamp: 8318
    > [ 17.892746] hardirqs last enabled at (8317): [<c01032c8>] restore_nocheck+0x12/0x15
    > [ 17.906778] hardirqs last disabled at (8318): [<c0103203>] sysenter_past_esp+0x6c/0x99
    > [ 17.921481] softirqs last enabled at (7128): [<c0123cd1>] __do_softirq+0xe9/0xfa
    > [ 17.936962] softirqs last disabled at (7121): [<c0123d3e>] do_softirq+0x5c/0x60

    this means that the last registered 'hardirqs off' event was
    sysenter_past_esp, i.e. the normal sysenter syscall entry code - but
    nothing re-enabled hardirqs - which is weird, given that you ended up in
    sys_brk().

    > I've replaced the DEBUG_LOCKS_WARN_ON by a BUG, and it appears that
    > the user space program calling sys_brk is hotplug.

    (ok, i'll enhance the debug printout to include the process name and
    PID.)

    Ingo
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