linux-kernel - Re: [lockdep] b09be676e0 BUG: unable to handle kernel NULL pointer dereference at 000001f2

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Date:   Tue, 10 Oct 2017 14:08:42 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Fengguang Wu <fengguang.wu@...el.com>,
        Ingo Molnar <mingo@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        LKP <lkp@...org>, Josh Poimboeuf <jpoimboe@...hat.com>,
        kernel-team@....com
Subject: Re: [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@...el.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.