[question] Can the execution of the atomtic operation instruction pair lwarx/stwcx be interrrupted by local HW interruptions?

[wyang]

On 01/06/2014 02:24 PM, Gavin Hu wrote:
> So, these primitive funcitons like atomic_add() and so on also can't 
> prevent processes schedule switch on local CPU core? right?

You are right!

BR
Wei
>
> Thanks!
>
>
> BR
> Gvain. Hu
>
>
> On Mon, Jan 6, 2014 at 1:27 PM, wyang <w90p710 at gmail.com 
> <mailto:w90p710 at gmail.com>> wrote:
>
>
>     On 01/06/2014 11:41 AM, Gavin Hu wrote:
>>     Thanks your response.  :)
>>     But that means that these optimitive operations like atomic_add()
>>     aren't optimitive actully in PPC architecture, right? Becuase
>>     they can be interrupted by loacl HW interrupts. Theoretically,
>>     the ISR also can access the atomic gloable variable.
>
>     Nope, my understand is that if you wanna sync kernel primitive
>     code with ISR, you have responsibility to disable local
>     interrupts. atomic_add does not guarantee to handle such case.
>
>     Thanks
>     Wei
>
>
>>
>>
>>     The following codes are complete atomic_inc() copied from arch/
>>     static __inline__ void atomic_add(int a, atomic_t *v)
>>     {
>>         int t;
>>
>>         __asm__ __volatile__(
>>     "1:    lwarx    %0,0,%3        # atomic_add\n\
>>         add    %0,%2,%0\n"
>>         PPC405_ERR77(0,%3)
>>     "    stwcx.    %0,0,%3 \n\
>>         bne-    1b"
>>         : "=&r" (t), "+m" (v->counter)
>>         : "r" (a), "r" (&v->counter)
>>         : "cc");
>>     }
>>
>>
>>     BR
>>     Gavin. Hu
>>
>>
>>     On Mon, Dec 30, 2013 at 9:54 AM, wyang <w90p710 at gmail.com
>>     <mailto:w90p710 at gmail.com>> wrote:
>>
>>         On 12/28/2013 01:41 PM, Gavin Hu wrote:
>>>         Hi
>>>
>>>         I notice that there is a pair ppc instructions lwarx and
>>>         stwcx used to atomtic operation for instance,
>>>         atomic_inc/atomic_dec.
>>>
>>>         In some ppc manuals, they more emphasize its mechanism is
>>>         that lwarx can reseve the target memory address preventing
>>>         other CORE from modifying it.
>>>
>>>         I assume that there is atomtic operation executing on the
>>>         CORE0 in a multicore system. In this situation, does the
>>>         CORE0 disable the local HW interrupt?
>>>         Can the executing process from the beginning of lwarx and
>>>         end of stwcx be interrupted by HW interruptions/exceptions? 
>>>         Anyway, they are two assembly instructions.
>>
>>         It should just like other arch, the processor should response
>>         any interrupt after the execution of a instruction, so the
>>         local HW interrupt is not disabled.
>>
>>         Thanks
>>         Wei
>>>
>>>          Thanks a lot!
>>>
>>>         "1:    lwarx    %0,0,%2        # atomic_inc\n\
>>>             addic    %0,%0,1\n"
>>>         "    stwcx.    %0,0,%2 \n\
>>>
>>>
>>>         BR
>>>         Gavin. Hu
>>>
>>>
>>>         _______________________________________________
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>>>         Linuxppc-dev at lists.ozlabs.org  <mailto:Linuxppc-dev at lists.ozlabs.org>
>>>         https://lists.ozlabs.org/listinfo/linuxppc-dev
>>
>>
>
>

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