Re: [PATCH 2/2] printk: wake up klogd in vprintk_emit

[Petr Mladek]

On Wed 2018-04-18 10:29:43, Steven Rostedt wrote:
> On Sat, 14 Apr 2018 12:01:45 +0900
> Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx> wrote:
> 
> > --- a/kernel/printk/printk.c
> > +++ b/kernel/printk/printk.c
> > @@ -1888,6 +1888,7 @@ asmlinkage int vprintk_emit(int facility, int level,
> >  
> >  	printed_len = log_output(facility, level, lflags, dict, dictlen, text, text_len);
> >  
> > +	wake_up_klogd();
> >  	logbuf_unlock_irqrestore(flags);
> 
> You can't do this, because the scheduler can call printk_deferred()
> with the rq lock held, and printk_deferred() will grab the logbuf lock.
> 
> Calling wake_up_klogd() will grab the rq lock and give us a A-B<->B-A
> locking order.

wake_up_klogd() uses the lockless irq_work_queue(). So it is actually
safe.

But the name is confusing. We should rename it.

Best Regards,
Petr