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    SubjectRe: 2.6.18-rt1
    On Fri, Oct 13, 2006 at 11:18:01PM +0200, Karsten Wiese wrote:
    > Am Samstag, 30. September 2006 20:18 schrieb Dipankar Sarma:
    > > On Sat, Sep 30, 2006 at 02:06:04PM -0400, Lee Revell wrote:
    > > > On Wed, 2006-09-20 at 16:19 +0200, Ingo Molnar wrote:
    > > > > I'm pleased to announce the 2.6.18-rt1 tree, which can be downloaded
    > > > > from the usual place:
    > > > >
    > > > > http://redhat.com/~mingo/realtime-preempt/
    > > >
    > > > I got this Oops with -rt3, looks RCU related. Apologies in advance if
    > > > it's already known.
    > > >
    > > > Unable to handle kernel NULL pointer dereference at 0000000000000000 RIP:
    > > > [<ffffffff802aafa7>] __rcu_read_unlock+0x2e/0x82
    > > > PGD 46a3067 PUD 4e27067 PMD 0
    > > > Oops: 0002 [1] PREEMPT SMP
    > > > CPU 1
    > >
    > > I see a very similar crash while running rcutorture on 2.6.18-mm1 and
    > > my rcu patchset that has rcupreempt stuff rom -rt. I don't see this
    > >
    >
    > Bug just happened here on a tainted UP x86_64 running rt4.
    > IIRC this is the second time in 2 weeks or so.
    > Machine seams to be fine still after the oops...
    >
    > <Oops>
    > Unable to handle kernel NULL pointer dereference at 0000000000000000 RIP:
    > [<ffffffff802a1b21>] __rcu_read_unlock+0x2e/0x80
    > PGD 3b616067 PUD 1718b067 PMD 0
    > Oops: 0002 [1] PREEMPT
    > CPU 0
    > Modules linked in: autofs4 sunrpc video button ac lp parport_pc parport nvram snd_via82xx gameport snd_ac97_codec snd_ac97_bus snd_seq_dummy snd_seq_oss snd_seq_midi_event snd_seq nvidia snd_pcm_oss snd_mixer_oss snd_pcm ehci_hcd uhci_hcd snd_timer snd_page_alloc snd_mpu401_uart snd_rawmidi pcspkr snd_seq_device snd i2c_viapro i2c_core r8169 soundcore ext3 jbd
    > Pid: 7102, comm: sh Tainted: P 2.6.18-rt4 #4
    > RIP: 0010:[<ffffffff802a1b21>] [<ffffffff802a1b21>] __rcu_read_unlock+0x2e/0x80


    Sorry, I should have published my investigations long ago. I tracked
    this down (atleast the crash in my machine) to NMI interference
    with rcu_read_lock()/rcu_read_unlock(). We use those APIs
    from NMI context as well
    (default_do_nmi()->notify_die()->atomic_notifier_call_chain()).

    Can you try with nmi_watchdog=0 in the kernel command line ?

    Paul has an NMI-safe patch for rcupreempt which I am adopting
    and testing at the moment. If this works well, I will publish
    a new patchset.

    Thanks
    Dipankar
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