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

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Date:	Fri, 21 Jan 2011 13:10:51 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>, Tejun Heo <tj@...nel.org>,
	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

* Christoph Lameter (cl@...ux.com) wrote:
> 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.

Isn't that a bit much to try to match the type of each oldword/newword
parameter to the structure fields ? Having separated word 1-2 parameter is just
an artefact caused by the inability of some gcc to deal with int128; were we to
use int128, we would have none of this type-checking whatsoever.

We could simply check that the first parameter alignment is >= 2 * sizeof(long)
and that its size == 2 * sizeof(long), so that the layout in memory fits the
cmpxchg_double requirements. This should work both for structure and array
parameters.

Now if the user needs to map "oldword1, oldword2" to the actual percpu_dd
fields, we could ensure that the order of these two parameters actually match
the structure field or array index order. This would, of course, be documented
above this_cpu_cmpxchg_double().

> 
> > 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.

"if the pointer passed is constant" -> if you use the actual type of percpu_dd
to check the alignment, then you can do an alignment check at compile-time even
for a non-const parameter. The requirement imposed on typing will take care to
make sure that even a non-const pointer will have the proper alignment.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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