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

Sergey Senozhatsky
2018-10-22T10:09:52Z

On (10/21/18 11:09), Daniel Wang wrote:
>
> Just got back from vacation. Thanks for the continued discussion. Just so
> I understand the current state. Looks like we've got a pretty good explanation
> of what's going on (though not completely sure), and backporting Steven's
> patches is still the way to go?

Up to -stable maintainers.

Note, with or without Steven's patch, the non-reentrable consoles are
still non-reentrable, so the deadlock is still there:

spin_lock_irqsave(&port->lock, flags)
<NMI>
panic()
console_flush_on_panic()
spin_lock_irqsave(&port->lock, flags) // deadlock

// And I wouldn't mind to have more reviews/testing on [1].

Another deadlock scenario could be the following one:

printk()
console_trylock()
down_trylock()
raw_spin_lock_irqsave(&sem->lock, flags)
<NMI>
panic()
console_flush_on_panic()
console_trylock()
raw_spin_lock_irqsave(&sem->lock, flags) // deadlock

There are no patches addressing this one at the moment. And it's
unclear if you are hitting this scenario.

> I see that Sergey had sent an RFC series for similar things. Are those
> trying to solve the deadlock problem in a different way?

Umm, I wouldn't call it "another way". It turns non-reentrant serial
consoles to re-entrable ones. Did you test patch [1] from that series
on you environment, by the way?

[1] lkml.kernel.org/r/20181016050428.17966-2-sergey.senozhatsky@gmail.com

-ss