Subject: Re: possible circular locking dependency detected [was: linux-next: Tree for Aug 22]

From: Boqun Feng <[email-masked]>
Date: Wed, 23 Aug 2017 13:35:01 +0800

On Wed, Aug 23, 2017 at 01:46:48PM +0900, Sergey Senozhatsky wrote:
> On (08/23/17 12:38), Boqun Feng wrote:
> [..]
> > diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> > index 642fb5362507..a3709e15f609 100644
> > --- a/kernel/locking/lockdep.c
> > +++ b/kernel/locking/lockdep.c
> > @@ -1156,6 +1156,23 @@ print_circular_lock_scenario(struct held_lock *src,
> >  		__print_lock_name(target);
> >  		printk(KERN_CONT ");\n");
> 
> KERN_CONT and "\n" should not be together. "\n" flushes the cont
> buffer immediately.
> 

Hmm.. Not quite familiar with printk() stuffs, but I could see several
usages of printk(KERN_CONT "...\n") in kernel.

Did a bit research myself, and I now think the inappropriate use is to
use a KERN_CONT printk *after* another printk ending with a "\n". Am I
missing some recent changes or rules of KERN_CONT?

Regards,
Boqun

> 	-ss
> 
> >  		printk("\n *** DEADLOCK ***\n\n");
> > +	} else if (cross_lock(src->instance)) {
> > +		printk(" Possible unsafe locking scenario by crosslock:\n\n");
> > +		printk("       CPU0                    CPU1\n");
> > +		printk("       ----                    ----\n");
> > +		printk("  lock(");
> > +		__print_lock_name(target);
> > +		printk(KERN_CONT ");\n");
> > +		printk("  lock(");
> > +		__print_lock_name(source);
> > +		printk(KERN_CONT ");\n");
> > +		printk("                               lock(");
> > +		__print_lock_name(parent == source ? target : parent);
> > +		printk(KERN_CONT ");\n");
> > +		printk("                               unlock(");
> > +		__print_lock_name(source);
> > +		printk(KERN_CONT ");\n");
> > +		printk("\n *** DEADLOCK ***\n\n");
> >  	} else {
> >  		printk(" Possible unsafe locking scenario:\n\n");
> >  		printk("       CPU0                    CPU1\n");
> > -- 
> > 2.14.1
> >