linux-kernel - Re: lockdep warning: console vs. mem hotplug

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Date:   Sat, 25 Mar 2017 10:37:50 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Sebastian Ott <sebott@...ux.vnet.ibm.com>,
        Petr Mladek <pmladek@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.org>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: lockdep warning: console vs. mem hotplug

On (03/24/17 21:08), Steven Rostedt wrote:
> > Sebastian, does this change make lockdep happy?
> > 
> > it removes console drivers from the __offline_isolated_pages(). not the
> > best solution I can think of, but the simplest one.
> > 
> > ---
> > 
> >  mm/page_alloc.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index f749b7ff7c50..eb61e6ab5f4f 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -7705,7 +7705,7 @@ __offline_isolated_pages(unsigned long start_pfn, unsigned long end_pfn)
> >  		BUG_ON(!PageBuddy(page));
> >  		order = page_order(page);
> >  #ifdef CONFIG_DEBUG_VM
> > -		pr_info("remove from free list %lx %d %lx\n",
> > +		printk_deferred(KERN_INFO "remove from free list %lx %d %lx\n",
> >  			pfn, 1 << order, end_pfn);
> >  #endif
> >  		list_del(&page->lru);
> 
> 
> My fear is that this will trigger for any printk in page_alloc.c under
> the zone lock.

absolutely true.

I Cc'd debugobjects, mm and sclp_console maintainers because the
real (smart) solution to the problem is somewhere there.



another problem (not reported) is that we have conflicting dependencies
	mod_timer -> sclp_console
	sclp_console -> mod_timer

which can result in a deadlock:
	mod_timer -> debugobjects -> printk -> sclp_console -> mod_timer


this one, I think, can be addressed by switching to a printk_safe in debugobjects.


// I'm traveling now. there will be delays in replies, sorry.

	-ss