linux-kernel - Re: [cpuops cmpxchg double V2 1/4] Generic support for this_cpu_cmpxchg

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Date:	Fri, 21 Jan 2011 12:06:45 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
cc:	Tejun Heo <tj@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [cpuops cmpxchg double V2 1/4] Generic support for
 this_cpu_cmpxchg_double

On Fri, 21 Jan 2011, Mathieu Desnoyers wrote:

> > > 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.
>
> So the first argument could be along the lines of:
>
> struct cmpxchg_double pcp
>
> then.

Ok then you would pass a struct by value? Or use a non-scalar as a
variable passed to a this_cpu_op? So far per cpu scalars have been the
only variables allowed to be specified in this_cpu operations.

> > >         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?
>
> Well, my last proposal to check __alignof__ within a __builtin_choose_expr
> check wouldn't need this union actually, which would be much better I think.

The existing implementation has a check for alignment. That is not the
problem. The typechecking would need to be addressed. I.e. if I pass a
pointer for old and an ulong for the new value then I'd like to see the
compiler complain. Or if the first parameter is a long but the type of the
first word is a pointer etc etc.
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