Re: 4.14 backport request for dbdda842fe96f: "printk: Add console owner and waiter logic to load balance console writes"

Petr Mladek
2018-10-04T08:36:09Z

On Thu 2018-10-04 16:44:42, Sergey Senozhatsky wrote:
> On (10/03/18 11:37), Daniel Wang wrote:
> > When `softlockup_panic` is set (which is what my original repro had and
> > what we use in production), without the backport patch, the expected panic
> > would hit a seemingly deadlock. So even when the machine is configured
> > to reboot immediately after the panic (kernel.panic=-1), it just hangs there
> > with an incomplete backtrace. With your patch, the deadlock doesn't happen
> > and the machine reboots successfully.
> >
> > This was and still is the issue this thread is trying to fix. The last
> > log snippet
> > was from an "experiment" that I did in order to understand what's really
> > happening. So far the speculation has been that the panic path was trying
> > to get a lock held by a backtrace dumping thread, but there is not enough
> > evidence which thread is holding the lock and how it uses it. So I set
> > `softlockup_panic` to 0, to get panic out of the equation. Then I saw that one
> > CPU was indeed holding the console lock, trying to write something out. If
> > the panic was to hit while it's doing that, we might get a deadlock.
>
> Hmm, console_sem state is ignored when we flush logbuf, so it's OK to
> have it locked when we declare panic():
>
> void console_flush_on_panic(void)
> {
> /*
> * If someone else is holding the console lock, trylock will fail
> * and may_schedule may be set. Ignore and proceed to unlock so
> * that messages are flushed out. As this can be called from any
> * context and we don't want to get preempted while flushing,
> * ensure may_schedule is cleared.
> */
> console_trylock();
> console_may_schedule = 0;
> console_unlock();
> }
>
> Things are not so simple with uart_port lock. Generally speaking we
> should deadlock when we NMI panic() kills the system while one of the
> CPUs holds uart_port lock.

This looks like a reasonable explanation of what is happening here.
It also explains why the console owner logic helped.

> 8250 has sort of a workaround for this scenario:
>
> serial8250_console_write()
> {
> if (port->sysrq)
> locked = 0;
> else if (oops_in_progress)
> locked = spin_trylock_irqsave(&port->lock, flags);
> else
> spin_lock_irqsave(&port->lock, flags);
>
> ...
> uart_console_write(port, s, count, serial8250_console_putchar);
> ...
>
> if (locked)
> spin_unlock_irqrestore(&port->lock, flags);
> }
>
> Now... the problem. A theory, in fact.
> panic() sets oops_in_progress back to zero - bust_spinlocks(0) - too soon.

I see your point. I am just a bit scared of this way. Ignoring locks
is a dangerous and painful approach in general.

Best Regards,
Petr