Subject: Re: WARNING: at mm/slub.c:3357, kernel BUG at mm/slub.c:3413

From: Benjamin Herrenschmidt <[email-masked]>
Date: Wed, 23 Nov 2011 08:58:05 +1100

On Tue, 2011-11-22 at 08:48 +0100, Eric Dumazet wrote:
> Le lundi 21 novembre 2011 à 21:18 -0600, Christoph Lameter a écrit :
> 
> > Hmmm... That means that c->page points to page not frozen. Per cpu
> > partial pages are frozen until they are reused or until the partial list
> > is flushed.
> > 
> > Does this ever happen on x86 or only on other platforms? In put_cpu_partial() the
> > this_cpu_cmpxchg really needs really to be irq safe. this_cpu_cmpxchg is
> > only preempt safe.
> > 
> > Index: linux-2.6/mm/slub.c
> > ===================================================================
> > --- linux-2.6.orig/mm/slub.c	2011-11-21 21:15:41.575673204 -0600
> > +++ linux-2.6/mm/slub.c	2011-11-21 21:16:33.442336849 -0600
> > @@ -1969,7 +1969,7 @@
> >  		page->pobjects = pobjects;
> >  		page->next = oldpage;
> > 
> > -	} while (this_cpu_cmpxchg(s->cpu_slab->partial, oldpage, page) != oldpage);
> > +	} while (irqsafe_cpu_cmpxchg(s->cpu_slab->partial, oldpage, page) != oldpage);
> >  	stat(s, CPU_PARTIAL_FREE);
> >  	return pobjects;
> >  }
> > 
> 
> For x86, I wonder if our !X86_FEATURE_CX16 support is correct on SMP
> machines.
> 
> this_cpu_cmpxchg16b_emu() claims to be IRQ safe, but may be buggy...
> 
> Could we have somewhere a NMI handler calling kmalloc() ?

Christian and I are on ppc, which uses the generic implementation of
this_cpu_cmpxchg() which is not irq safe. So the above patch is needed
regardless.

Christian, can you try it see if that helps in your case ?

Cheers,
Ben.

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