Re: [cpuops cmpxchg double V2 1/4] Generic support for this_cpu_cmpxchg_double

["H. Peter Anvin" <hpa@zytor.com>]

What is the problem with passing 128-bit structures by value?  I'm quite sure *that* is okay.

"Mathieu Desnoyers" <mathieu.desnoyers@efficios.com> wrote:

>* Christoph Lameter (cl@linux.com) wrote:
>> On Fri, 21 Jan 2011, H. Peter Anvin wrote:
>> 
>> > We could do cmpxchg with a structure... the problem with a lon int
>type is that Cristoph ran into bugs with __int128 on 64 bits.
>> 
>> We also would need to be pass the structure by value (well its really
>a
>> variable but its like passing by value) in order to be similar to the
>> other this_cpu_ops
>> 
>> You'd want either
>> 
>> DEFINE_PERCPU(struct mycustomdoublestruct, percpu_dd)
>> 
>> this_cpu_cmpxchg_double(percpu_dd, oldword1, oldword2, newword1,
>newword2)
>> 
>> with the problem of type checking
>
>What is the problem with type checking here ?
>
>We could use a gcc builtin like the following to check if the alignment
>of
>"percpu_dd" meets the double-cas requirements:
>
>#define testmacro(a, b) \
>   __builtin_choose_expr(__alignof__(a) >= 2 * sizeof(unsigned long), \
>                              ((a).low) = (b), \    /* success */
>                              ((a).low) = (void) 0) /* compile-error */
>
>> or
>> 
>> this_cpu_cmpxchg_double(percpu_dd, old_dd, new_dd)
>> 
>> with the problem of 128 bit constants/structs passed by value.
>
>Yeah, I guess trying to deal with 128-bit value might be a bit tricky.
>But
>having something sane and with compile-time-checked alignment for the
>percpu_dd
>type is not to be looked over.
>
>Thanks,
>
>Mathieu
>
>-- 
>Mathieu Desnoyers
>Operating System Efficiency R&D Consultant
>EfficiOS Inc.
>http://www.efficios.com

-- 
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