Re: printk deadlock due to double lock attempt on current CPU's runqueue

Peter Zijlstra
2021-11-10T11:20:50Z

On Wed, Nov 10, 2021 at 11:50:38AM +0100, Petr Mladek wrote:
> On Tue 2021-11-09 12:06:48, Sultan Alsawaf wrote:
> > Hi,
> >
> > I encountered a printk deadlock on 5.13 which appears to still affect the latest
> > kernel. The deadlock occurs due to printk being used while having the current
> > CPU's runqueue locked, and the underlying framebuffer console attempting to lock
> > the same runqueue when printk tries to flush the log buffer.
> >
> > I'm not sure what the *correct* solution is here (don't use printk while having
> > a runqueue locked? don't use schedule_work() from the fbcon path? tell printk
> > to use one of its lock-less backends?), so I've cc'd all the relevant folks.
>
> At the moment, printk_deferred() could be used here. It defers the
> console handling via irq_work().

I think I've rejected that patch at least twice now :-) John's printk
stuff will really land real soon now, right.