Re: [PATCH]atomic_inc_return() for i386/x86_64 (Re: RCU issue with SELinux)

[Kaigai Kohei]

Hi Andi, thanks for your comment.
Sorry, I have not noticed your mail in the flood of Linux-Kernel ML.

> > atomic_inc_return() is not defined for arm,arm26,i386,x86_64 and um archtectures.
> > This attached patch adds atomic_inc_return() and atomic_dec_return() to arm,i386 and x86_64.
> >
> > It is implemented by 'xaddl' operation with LOCK prefix for i386 and x86_64.
> > But this operation is permitted after i486 processor only.
> > Another implementation may be necessary for i386SX/DX processor.
> > But 'xaddl' operation is used in 'include/asm-i386/rwsem.h' unconditionally.
> > I think it has agreed on using 'xaddl' operation in past days.
> 
> We don't support SMP on 386 boxes. What you can do for 386 is to use 
> alternative() and just use an non SMP safe version for 386 and xadd 
> for 486+ 

We can avoid the problem by the simple solution, since SMP
on 386 boxes isn't supported. It is to disable interrupt
while updating atomic_t variable.

The attached patch modifies the include/asm-i386/atomic.h.
If the target processor is 386, then atomic_add_return() use
non-atomic operations between local_irq_disable() and local_irq_enable().
Otherwise, atomic_add_return() use 'xadd' operation with LOCK prefix.

By the way, do you know why 'xadd' operation is used
unconditionally in 'include/asm-i386/rwsem.h'?

Thanks.

Signed-off-by: KaiGai, Kohei <kaigai@xxxxxxxxxxxxx>
Signed-off-by: Takayoshi Kochi <t-kochi@xxxxxxxxxxxxx>
--------
Kai Gai <kaigai@xxxxxxxxxxxxx>

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