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    SubjectRe: lockdep: Warning & NULL ptr deref
    On Tue, Feb 28, 2012 at 3:29 PM, Peter Zijlstra <peterz@infradead.org> wrote:
    > On Tue, 2012-02-28 at 10:24 -0500, Sasha Levin wrote:
    >>
    >> I'll send some patches to prevent the NULL deref itself which happens
    >> since a chunk of the code in lockdep.c assumed hlock_class() can't
    >> return NULL.
    >>
    > There's tons of code actually assuming that..
    >
    >> This won't fix the actual problem though, I'm not sure exactly how a
    >> held lock wouldn't have a class initialized to it.
    >
    > static inline struct lock_class *hlock_class(struct held_lock *hlock)
    > {
    >        if (!hlock->class_idx) {
    >                /*
    >                 * Someone passed in garbage, we give up.
    >                 */
    >                DEBUG_LOCKS_WARN_ON(1);
    >                return NULL;
    >        }
    >        return lock_classes + hlock->class_idx - 1;
    > }
    >
    > Typically that translates to severe memory corruption.

    I can also reproduce it easily by triggering sysrq once or twice after
    trinity has stopped.
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