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    SubjectRe: [PATCH] kernel/watchdog: flush all printk nmi buffers when hardlockup detected
    On Wed 2020-02-12 10:15:51, Sergey Senozhatsky wrote:
    > On (20/02/10 12:48), Konstantin Khlebnikov wrote:
    > >
    > > In NMI context printk() could save messages into per-cpu buffers and
    > > schedule flush by irq_work when IRQ are unblocked. This means message
    > > about hardlockup appears in kernel log only when/if lockup is gone.
    > >
    > > Comment in irq_work_queue_on() states that remote IPI aren't NMI safe
    > > thus printk() cannot schedule flush work to another cpu.
    > >
    > > This patch adds simple atomic counter of detected hardlockups and
    > > flushes all per-cpu printk buffers in context softlockup watchdog
    > > at any other cpu when it sees changes of this counter.
    >
    > Petr, could you remind me, why do we do PRINTK_NMI_DIRECT_CONTEXT_MASK
    > only from ftrace?

    There was a possible deadlock when printing backtraces from all CPUs.
    The CPUs were serialized via a lock in nmi_cpu_backtrace(). One of
    them might have been interrupted under logbuf_lock.

    ftrace was needed because it printed too many messages. And it was
    safe because the ftrace log was read from a single CPU without
    any lock.

    Best Regards,
    Petr

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