Messages in this thread | | | Date | Wed, 12 Feb 2020 15:18:32 +0100 | From | Petr Mladek <> | Subject | Re: [PATCH] kernel/watchdog: flush all printk nmi buffers when hardlockup detected |
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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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