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

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Date:	Fri, 21 Jan 2011 12:50:54 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Christoph Lameter <cl@...ux.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

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

So the first argument could be along the lines of:

struct cmpxchg_double pcp

then.

> You would be implicitly passing a 128 bit argument although the
> compiler may not need to generate code for that.

Sorry, I don't understand this last statement. Does it still apply if we pass
pcp as I just proposed ? (without the pointer, with a __builtin_choose_expr
check on __alignof__ of the pcp parameter)

> 
> > 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?

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.

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

I'm going back to the points that have been previously dismissed with a rather
large degree of handwaving. ;) I'm OK with looking into the API and providing
code snippets to show the basic implementation behind the ideas, but I
unfortunately don't have the bandwidth to stop everything I'm currently working
on and start working on double-cas patches. Sorry, I wish I could do more.

Thanks,

Mathieu


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