Re: [PATCH] kernel/watchdog: flush all printk nmi buffers when hardlockup detected

[Petr Mladek]

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