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Date: Fri, 21 Jan 2011 11:50:29 -0600 (CST)
From: Christoph Lameter <cl@linux.com>
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To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
cc: "H. Peter Anvin" <hpa@zytor.com>, Tejun Heo <tj@kernel.org>,
        Pekka Enberg <penberg@cs.helsinki.fi>, akpm@linux-foundation.org,
        linux-kernel@vger.kernel.org, Eric Dumazet <eric.dumazet@gmail.com>
Subject: Re: [cpuops cmpxchg double V2 1/4] Generic support for
 this_cpu_cmpxchg_double
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On Fri, 21 Jan 2011, Mathieu Desnoyers wrote:

> > this_cpu_cmpxchg_double(percpu_dd, oldword1, oldword2, newword1, newword2)
> >
> > with the problem of type checking
>
> What is the problem with type checking here ?

You need to know the fields in the struct to do the type checking with
each of the other parameters.

> 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.

The existing implementation could be equipped to do a compile time check
for the proper alignment if the pointer passed is constant.