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    SubjectRe: [debug patch] printk: Add a printk killswitch to robustify NMI watchdog messages

    * Arne Jansen <> wrote:

    > > hm, it's hard to interpret that without the spin_lock()/unlock()
    > > logic keeping the dumps apart.
    > The locking was in place from the beginning. [...]

    Ok, i was surprised it looked relatively ordered :-)

    > [...] As the output is still scrambled, there are other sources for
    > BUG/WARN outside the watchdog that trigger in parallel. Maybe we
    > should protect the whole BUG/WARN mechanism with a lock and send it
    > to early_printk from the beginning, so we don't have to wait for
    > the watchdog to kill printk off and the first BUG can come through.
    > Or just let WARN/BUG kill off printk instead of the watchdog
    > (though I have to get rid of that syslog-WARN on startup).

    I had yet another look at your lockup.txt and i think the main cause
    is the WARN_ON() caused by the not-held pi_lock. The lockup there
    causes other CPUs to wedge in printk, which triggers spinlock-lockup
    messages there.

    So i think the primary trigger is the pi_lock WARN_ON() (as your
    bisection has confirmed that too), everything else comes from this.

    Unfortunately i don't think we can really 'fix' the problem by
    removing the assert. By all means the assert is correct: pi_lock
    should be held there. If we are not holding it then we likely won't
    crash in an easily visible way - it's a lot easier to trigger asserts
    than to trigger obscure side-effects of locking bugs.

    It is also a mystery why only printk() triggers this bug. The wakeup
    done there is not particularly special, so by all means we should
    have seen similar lockups elsewhere as well - not just with
    printk()s. Yet we are not seeing them.

    So some essential piece of the puzzle is still missing.



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