On Wed, 4 May 2011, Ingo Molnar wrote:
> * 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.

SMP is not an issue because that's cpu local access, but it's
irq/softirq unsafe. See my other mail.

Thanks,

	tglx