Re: deadlock caused by printk

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Fri, 2011-03-25 at 20:15 +0800, Ming Lei wrote:
> Hi,
> 
> These days I was studying "perf code" and try to dump some debug
> info in event_sched_out(): kernel/perf_event.c by printk, but found
> system can hang easily.
> 
> After digging into related code for several days, I find the hang is
> caused by spinlock deadlock when acquiring rq->lock in task_rq_lock(),
> which is called in the path below:
> 
>           try_to_wake_up<-wake_up_process<-up<-console_unlock<-printk.
> 
> I wonder if there are some usage conventions about printk, which says
> explicitly that printk can not by used in some places of kernel. Or does the
> above show that it is a bug of kernel?
> 
> If the above is really a limitation of printk, have we other dump functions
> which may be used safely in some places of kernel source code where
> printk is not allowed.

Don't use printk from NMI context, that's never going to work well.

One thing you could to is use trace_printk() and use ftrace to debug
perf.