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Date: Fri, 21 Jan 2011 11:07:16 -0600 (CST)
From: Christoph Lameter <cl@linux.com>
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To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
cc: Tejun Heo <tj@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
        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:

> I'm again probably missing something, but what is "clumsy" about defining a
> structure like the following to ensure proper alignment of the target
> pointer (instead of adding a runtime test) ?
>
> struct cmpxchg_double {
> #if __BYTE_ORDER == __LITTLE_ENDIAN
>         unsigned long low, high;
> #else
>         unsigned long high, low;
> #endif
> } __attribute__((packed, aligned(2 * sizeof(unsigned long))));
>
> (note: packed here along with "aligned" does _not_ generate ugly bytewise
> read/write memory ops like "packed" alone. The use of "packed" is to let the
> compiler down-align the structure to the value requested, instead of uselessly
> aligning it on 32-byte if it chooses to.)
>
> The prototype could then look like:
>
> bool __this_cpu_generic_cmpxchg_double(pcp, oval_low, oval_high, nval_low, nval_high);
>
> With:
>   struct cmpxchg_double *pcp

That does not conform to the parameter conventions in other this_cpu_ops.
The first parameter is a variable because the notion of a pointer is
problematic given that percpu operations use a segment prefix to relocate
pointers. You would be implicitly passing a 128 bit argument although the
compiler may not need to generate code for that.

> I think Christoph's point is that he wants to alias this with a pointer. Well,
> this can be done cleanly with:
>
> union {
>         struct cmpxchg_double casdbl;
>         struct {
>                 void *ptr;
>                 unsigned long cpuid_tid;
>         } t;
> }

There is no need for aliases with the existing implementation.

How will the macro check the parameters now?

> Thoughts ?

Could you actually try to write a patch instead running through points
that we have discussed earlier?