linux-kernel - Re: [PATCH][RT] Dereference pointer to cpu id, not to address of CPUID

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Date:	Sun, 09 Nov 2008 03:10:28 -0800
From:	Sven-Thorsten Dietrich <sven@...bigcorporation.com>
To:	Juergen Beisert <jbe@...gutronix.de>
Cc:	linux-rt-users@...r.kernel.org,
	LKML <Linux-kernel@...r.kernel.org>, Tony Jones <tonyj@...e.de>
Subject: Re: [PATCH][RT] Dereference pointer to cpu id, not to address of
	CPUID

On Sun, 2008-11-09 at 11:20 +0100, Juergen Beisert wrote:
> On Freitag, 7. November 2008, Sven-Thorsten Dietrich wrote:
> > This patch applies to 2.6.25-rt, 2.6.26-rt and 2.6.27-rt
> >
> > From: Sven-Thorsten Dietrich <sdietrich@...e.de>
> > Subject: Dereference pointer to cpu id, when evaluating condition.
> >
> > Without dereferencing, the condition always evaluates to true.
> >
> > Signed-off-by: Sven-Thorsten Dietrich <sdietrich@...e.de>
> > ---
> >  mm/slab.c |    2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > --- a/mm/slab.c
> > +++ b/mm/slab.c
> > @@ -2033,7 +2033,7 @@ slab_destroy(struct kmem_cache *cachep,
> >  	} else {
> >  		kmem_freepages(cachep, addr);
> >  		if (OFF_SLAB(cachep)) {
> > -			if (this_cpu)
> > +			if (*this_cpu)
> >  				__cache_free(cachep->slabp_cache, slabp, this_cpu);
> >  			else
> >  				kmem_cache_free(cachep->slabp_cache, slabp);
> 
> When I use this patch, I get the following (architecture is PowerPC MPC5200B):
> 
> Oops: Exception in kernel mode, sig: 5 [#1]
> PREEMPT ksp0058
> Modules linked in:
> NIP: c01bdda4 LR: c006f60c CTR: 00000000
> REGS: c1845db0 TRAP: 0700   Not tainted  (2.6.26.7-rt11-ptx-trunk)
> MSR: 00021032 <ME,IR,DR>  CR: 82002028  XER: 00000000
> TASK = c183b0b0[15] 'events/0' THREAD: c1844000
> GPR00: 00000001 c1845e60 c183b0b0 c028ac60 c1a23de0 00009032 c02ca680 c02d6000
> GPR08: c183b0b0 00000001 c028ac60 c183b0b0 c1a35000 ffffffff 01ffe000 ffffffff
> GPR16: 00000001 c027d000 c026019c c0260000 c026019c c1821f98 00000000 00000002
> GPR24: 00100100 00200200 c1800540 c028ac60 c1844000 c028ac60 c1802490 c1802480
> NIP [c01bdda4] rt_spin_lock_slowlock+0x5c/0x26c
> LR [c006f60c] kmem_cache_free+0x30/0x5c
> Call Trace:
> [c1845e60] [c01bbea0] preempt_schedule_irq+0x70/0xa0 (unreliable)
> [c1845ed0] [c006f60c] kmem_cache_free+0x30/0x5c
> [c1845f00] [c006fb58] drain_freelist+0x88/0x108
> [c1845f40] [c0070f4c] cache_reap+0x100/0x140
> [c1845f60] [c002fe84] run_workqueue+0x13c/0x240
> [c1845f90] [c0030620] worker_thread+0x74/0xd4
> [c1845fd0] [c0034468] kthread+0x48/0x84
> [c1845ff0] [c000fcdc] kernel_thread+0x44/0x60
> Instruction dump:
> 543c0024 813c000c 39290001 913c000c 80030004 2f800000 419e01f4 801b0010
> 5400003a 7c001278 7c000034 5400d97e <0f000000> 38800001 7f63db78 83220000
> Oops: Exception in kernel mode, sig: 5 [#2]
> PREEMPT ksp0058
> Modules linked in:
> NIP: c01bdda4 LR: c006f60c CTR: 00000000
> REGS: c1845ab0 TRAP: 0700   Tainted: G      D    (2.6.26.7-rt11-ptx-trunk)
> MSR: 00021032 <ME,IR,DR>  CR: 84008048  XER: 20000000
> TASK = c183b0b0[15] 'events/0' THREAD: c1844000
> GPR00: 00000001 c1845b60 c183b0b0 c028ac60 c1842580 00001032 c0260148 c0260144
> GPR08: c183b0b0 00000002 c028ac60 c183b0b0 c1835710 ffffffff 01ffe000 ffffffff
> GPR16: 00000001 c027d000 c026019c c0260000 c026019c c1821f98 c0020000 c0280000
> GPR24: c0280000 c1844000 c1842580 c028ac60 c1844000 c028ac60 c1835580 c183b0b0
> NIP [c01bdda4] rt_spin_lock_slowlock+0x5c/0x26c
> LR [c006f60c] kmem_cache_free+0x30/0x5c
> Call Trace:
> [c1845b60] [0000000f] 0xf (unreliable)
> [c1845bd0] [c006f60c] kmem_cache_free+0x30/0x5c
> [c1845c00] [c001aae0] __cleanup_sighand+0x34/0x44
> [c1845c10] [c001fefc] release_task+0x23c/0x3b4
> [c1845c50] [c00214b4] do_exit+0x5e8/0x66c
> [c1845c90] [c000de78] kernel_bad_stack+0x0/0x4c
> [c1845cb0] [c000e128] _exception+0x16c/0x180
> [c1845da0] [c00104e8] ret_from_except_full+0x0/0x4c
> --- Exception: 700 at rt_spin_lock_slowlock+0x5c/0x26c
>     LR = kmem_cache_free+0x30/0x5c
> [c1845e60] [c01bbea0] preempt_schedule_irq+0x70/0xa0 (unreliable)
> [c1845ed0] [c006f60c] kmem_cache_free+0x30/0x5c

The trace shows re-entrancy to kmem_cache_free+0x30

I suspect, that you are deadlocking on

spin_lock(&l3->list_lock);

in cache_flusharray. 

The task would already be holding the lock from the first pass through
kmem_cache_free.

That being said, I started seeing a similar deadlock on x86, where I am
triggering 

BUG_ON(rt_mutex_owner(lock) == current); 

in kernel/rtmutex.c:831

Still I have not convinced myself that my patch above is wrong.

Sven

> [c1845f00] [c006fb58] drain_freelist+0x88/0x108
> [c1845f40] [c0070f4c] cache_reap+0x100/0x140
> [c1845f60] [c002fe84] run_workqueue+0x13c/0x240
> [c1845f90] [c0030620] worker_thread+0x74/0xd4
> [c1845fd0] [c0034468] kthread+0x48/0x84
> [c1845ff0] [c000fcdc] kernel_thread+0x44/0x60
> Instruction dump:
> 543c0024 813c000c 39290001 913c000c 80030004 2f800000 419e01f4 801b0010
> 5400003a 7c001278 7c000034 5400d97e <0f000000> 38800001 7f63db78 83220000
> 
> It happens immediately after the system shows the login prompt.
> 
> jbe
> 
> -- 
> Dipl.-Ing. Juergen Beisert | http://www.pengutronix.de
>  Pengutronix - Linux Solutions for Science and Industry
>     Handelsregister: Amtsgericht Hildesheim, HRA 2686
>          Vertretung Sued/Muenchen, Germany
>    Phone: +49-8766-939 228 |  Fax: +49-5121-206917-9

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