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

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Date:   Tue, 28 Mar 2017 16:22:27 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     Sebastian Ott <sebott@...ux.vnet.ibm.com>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.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 Sat 25-03-17 09:04:42, Sergey Senozhatsky wrote:
> On (03/21/17 13:44), Sergey Senozhatsky wrote:
> [..]
> > so we probably can
> > 
> > 
> > 1) move pr_info() out of zone->lock in __offline_isolated_pages().
> >    meh...
> > 
> > 
> > 2) switch to printk_deferred() in __offline_isolated_pages().
> >    meh.. there might a bunch of other printks done from under zone->lock.
> > 
> > 
> > 3) move add_timer() out of sclp_con_lock console in sclp_console_write().
> >    well, there can be other consoles that do something similar.
> > 
> > 
> > 4) ... something smart.
> 
> 
> 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);

I believe this is not a proper fix. Although this code is ugly and maybe
it doesn't really need zone->lock because that should be the page
allocator internal thing the problem is that printk shouldn't impose
such a subtle dependency on locks. Why does the timer needs to allocate
at all?

-- 
Michal Hocko
SUSE Labs