linux-kernel - Re: Linux 2.6.25-rc2

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Date:	Tue, 19 Feb 2008 17:27:39 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Pekka Enberg <penberg@...helsinki.fi>,
	Torsten Kaiser <just.for.lkml@...glemail.com>,
	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Christoph Lameter <clameter@....com>
Subject: Re: Linux 2.6.25-rc2

On Tue, 19 Feb 2008 09:02:30 -0500
Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca> wrote:

> * Pekka Enberg (penberg@...helsinki.fi) wrote:
> > On Feb 19, 2008 8:54 AM, Torsten Kaiser <just.for.lkml@...glemail.com> wrote:
> > > > > [ 5282.056415] ------------[ cut here ]------------
> > > > > [ 5282.059757] kernel BUG at lib/list_debug.c:33!
> > > > > [ 5282.062055] invalid opcode: 0000 [1] SMP
> > > > > [ 5282.062055] CPU 3
> > > >
> > > > hm. Your crashes do seem to span multiple subsystems, but it always
> > > > seems to be around the SLUB code. Could you try the patch below? The
> > > > SLUB code has a new optimization and i'm not 100% sure about it. [the
> > > > hack below switches the SLUB optimization off by disabling the CPU
> > > > feature it relies on.]
> > > >
> > > >         Ingo
> > > >
> > > > ------------->
> > > >  arch/x86/Kconfig |    4 ----
> > > >  1 file changed, 4 deletions(-)
> > > >
> > > > Index: linux/arch/x86/Kconfig
> > > > ===================================================================
> > > > --- linux.orig/arch/x86/Kconfig
> > > > +++ linux/arch/x86/Kconfig
> > > > @@ -59,10 +59,6 @@ config HAVE_LATENCYTOP_SUPPORT
> > > >  config SEMAPHORE_SLEEPERS
> > > >         def_bool y
> > > >
> > > > -config FAST_CMPXCHG_LOCAL
> > > > -       bool
> > > > -       default y
> > > > -
> > > >  config MMU
> > > >         def_bool y
> > > >
> > >
> > > $ grep FAST_CMPXCHG_LOCAL */.config
> > > linux-2.6.24-rc2-mm1/.config:CONFIG_FAST_CMPXCHG_LOCAL=y
> > > linux-2.6.24-rc3-mm1/.config:CONFIG_FAST_CMPXCHG_LOCAL=y
> > > linux-2.6.24-rc3-mm2/.config:CONFIG_FAST_CMPXCHG_LOCAL=y
> > > linux-2.6.24-rc6-mm1/.config:CONFIG_FAST_CMPXCHG_LOCAL=y
> > > linux-2.6.24-rc8-mm1/.config:CONFIG_FAST_CMPXCHG_LOCAL=y
> > > linux-2.6.25-rc1/.config:CONFIG_FAST_CMPXCHG_LOCAL=y
> > > linux-2.6.25-rc2-mm1/.config:CONFIG_FAST_CMPXCHG_LOCAL=y
> > > linux-2.6.25-rc2/.config:CONFIG_FAST_CMPXCHG_LOCAL=y
> > >
> > > -rc2-mm1 still worked for me.
> > >
> > > Did you mean the new SLUB_FASTPATH?
> > > $ grep "define SLUB_FASTPATH" */mm/slub.c
> > > linux-2.6.25-rc1/mm/slub.c:#define SLUB_FASTPATH
> > > linux-2.6.25-rc2-mm1/mm/slub.c:#define SLUB_FASTPATH
> > > linux-2.6.25-rc2/mm/slub.c:#define SLUB_FASTPATH
> > >
> > > The 2.6.24-rc3+ mm-kernels did crash for me, but don't seem to contain this...
> > >
> > > On the other hand:
> > > From the crash in 2.6.25-rc2-mm1:
> > > [59987.116182] RIP  [<ffffffff8029f83d>] kmem_cache_alloc_node+0x6d/0xa0
> > >
> > > (gdb) list *0xffffffff8029f83d
> > > 0xffffffff8029f83d is in kmem_cache_alloc_node (mm/slub.c:1646).
> > > 1641                    if (unlikely(is_end(object) || !node_match(c, node))) {
> > > 1642                            object = __slab_alloc(s, gfpflags,
> > > node, addr, c);
> > > 1643                            break;
> > > 1644                    }
> > > 1645                    stat(c, ALLOC_FASTPATH);
> > > 1646            } while (cmpxchg_local(&c->freelist, object, object[c->offset])
> > > 1647
> > >  != object);
> > > 1648    #else
> > > 1649            unsigned long flags;
> > > 1650
> > >
> > > That code is part for SLUB_FASTPATH.
> > >
> > > I'm willing to test the patch, but don't know how fast I can find the
> > > time to do it, so my answer if your patch helps might be delayed until
> > > the weekend.
> > 
> > Mathieu, Christoph is on vacation and I'm not at all that familiar
> > with this cmpxchg_local() optimization, so if you could take a peek at
> > this bug report to see if you can spot something obviously wrong with
> > it, I would much appreciate that.
> 
> Sure,
> 
> Initial thoughts :
> 
> I'd like to get the complete config causing this bug. I suspect either :
> 
> - A race between the lockless algo and an IRQ in a driver allocating
>   memory.
> - stat(c, ALLOC_FASTPATH); seems to be using a var++, therefore
>   indicating it is not reentrant if IRQs are disabled. Since those are
>   only stats, I guess it's ok, but still weird.
> - CPU hotplug problem. 
>   http://bugzilla.kernel.org/attachment.cgi?id=14877&action=view shows
>   last sysfs file:
>   /sys/devices/system/cpu/cpu3/cache/index2/shared_cpu_map
>   -- is this linked to a cpu up/down event ?
> 
> Since this shows mostly with network card drivers, I think the most
> plausible cause would be an IRQ nesting over kmem_cache_alloc_node and
> calling it.
> 
> Will dig further...

I wonder how SLUB_FASTPATH is supposed to work, since it is affected by
a classical ABA problem of lockless algo.

cmpxchg_local(&c->freelist, object, object[c->offset]) can succeed,
while an interrupt came (on this cpu), and several allocations were done,
and one free was performed at the end of this interruption, so 'object'
was recycled.

c->freelist can then contain the previous value (object), but
object[c->offset] was changed by IRQ.

We then put back in freelist an already allocated object.

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