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

[Petr Mladek]

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().

There is no deferred variant for WARN() at the moment. The following
might work:

#define WARN_DEFERRED(condition, format...) ({		\
	unsigned long flags;				\
							\
	printk_safe_enter_irqsave(flags);		\
	WARN(condition, format...)			\
	printk_safe_exit_irqrestore(flags);		\
})

, where printk_safe_enter_irqsave()/printk_safe_exit_irqrestore(flags)
  are currently used only internally by printk() code and defined
  in the local kernel/printk/internal.h


Be ware that using the deferred variants is a whack a mole approach.
There are many printk() callers that might be called indirectly
and eventually cause deadlocks.

As already said, the plan is to upstream -rt solution and offload
the console work into kthreads. But it goes slowly. We want to
make it a clean way and prevent regressions as much as possible.

Best Regards,
Petr