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    SubjectRe: [lockdep] b09be676e0 BUG: unable to handle kernel NULL pointer dereference at 000001f2
    On Tue, Oct 03, 2017 at 09:54:31AM -0700, Linus Torvalds wrote:
    > On Tue, Oct 3, 2017 at 7:06 AM, Fengguang Wu <fengguang.wu@intel.com> wrote:
    > >
    > > This patch triggers a NULL-dereference bug at update_stack_state().
    > > Although its parent commit also has a NULL-dereference bug, however
    > > the call stack looks rather different. Both dmesg files are attached.
    > >
    > > It also triggers this warning, which is being discussed in another
    > > thread, so CC Josh. The full dmesg attached, too.
    > >
    > > Please press Enter to activate this console.
    > > [ 138.605622] WARNING: kernel stack regs at be299c9a in procd:340 has bad 'bp' value 000001be
    > > [ 138.605627] unwind stack type:0 next_sp: (null) mask:0x2 graph_idx:0
    > > [ 138.605631] be299c9a: 299ceb00 (0x299ceb00)
    > > [ 138.605633] be299c9e: 2281f1be (0x2281f1be)
    > > [ 138.605634] be299ca2: 299cebb6 (0x299cebb6)
    > >
    > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
    > >
    > > commit b09be676e0ff25bd6d2e7637e26d349f9109ad75
    > > locking/lockdep: Implement the 'crossrelease' feature

    Hello,

    I'm sorry for late. It was the biggest traditional holiday in Korea
    until yesterday.

    > Can we consider just reverting the crossrelease thing?
    >
    > The apparent stack corruption really worries me, and what worries me
    > most is that commit wasn't even supposed to change anything as far as
    > I can tell - it only adds infrastructure, no actual users that *set*
    > the cross-lock thing.

    All users of wait_for_completion() and lock_page() are implicitly actual
    users with CONFIG_LOCKDEP_CROSSRELEASE enabled, which sets the crosslock
    thing internally on its initialization.

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