Date: Wed, 4 May 2011 14:57:13 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Pekka Enberg <penberg@kernel.org>
Cc: Tejun Heo <tj@kernel.org>, Thomas Gleixner <tglx@linutronix.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Jens Axboe <axboe@kernel.dk>,
        Andrew Morton <akpm@linux-foundation.org>, werner <w.landgraf@ru.ru>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Christoph Lameter <cl@linux.com>
Subject: Re: [block IO crash] Re: 2.6.39-rc5-git2 boot crashs
Message-ID: <20110504125713.GA9978@elte.hu>
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 <alpine.LFD.2.02.1105041124080.3005@ionos>
 <20110504101932.GA3392@elte.hu>
 <alpine.LFD.2.02.1105041233280.3005@ionos>
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* Pekka Enberg <penberg@kernel.org> wrote:

> Hi Tejun,
> 
> On Wed, May 4, 2011 at 2:27 PM, Tejun Heo <tj@kernel.org> wrote:
> > Hmmm... if it's a race caused by preemtion enabled where it shouldn't
> > be, it's most likely the wrong type of this_cpu_cmpxchg_double() being
> > used in SLUB? ?ie. __this_cpu_cmpxchg_double() where it should have
> > been this_cpu_cmpxchg_double()? ?Christoph?
> 
> There's no __this_cpu_cmpxchg_double() usage in mm/slub.c so I don't
> think it's that simple.

Well, AFAICS the problem is:

earth4:~/tip> grep cmpxchg mm/slub.c 

		if (unlikely(!this_cpu_cmpxchg_double(
		if (unlikely(!this_cpu_cmpxchg_double(

Where this macro resolves to:

#  define this_cpu_cmpxchg_double_8(pcp1, pcp2, oval1, oval2, nval1, nval2)     \
        _this_cpu_generic_cmpxchg_double(pcp1, pcp2, oval1, oval2, nval1, nval2)

where:

#define _this_cpu_generic_cmpxchg_double(pcp1, pcp2, oval1, oval2, nval1, nval2)        \
({                                                                      \
        int ret__;                                                      \
        preempt_disable();                                              \
        ret__ = __this_cpu_generic_cmpxchg_double(pcp1, pcp2,           \
                        oval1, oval2, nval1, nval2);                    \
        preempt_enable();                                              
where:

#define __this_cpu_generic_cmpxchg_double(pcp1, pcp2, oval1, oval2, nval1, nval2)       \
({                                                                      \
        int __ret = 0;                                                  \
        if (__this_cpu_read(pcp1) == (oval1) &&                         \
                         __this_cpu_read(pcp2)  == (oval2)) {           \
                __this_cpu_write(pcp1, (nval1));                        \
                __this_cpu_write(pcp2, (nval2));                        \
                __ret = 1;                                              \
        }                                                               \
        (__ret);                                                        \
})

With is both IRQ and SMP unsafe.

Thanks,

	Ingo
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