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[Xen-devel] Re: [PATCH 1/4] x86: remove cast from void*

To: "H. Peter Anvin" <hpa@xxxxxxxxx>
Subject: [Xen-devel] Re: [PATCH 1/4] x86: remove cast from void*
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Tue, 14 Sep 2010 11:46:47 -0700
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx, Jeremy Fitzhardinge <jeremy@xxxxxxxxxxxxx>, matt mooney <mfm@xxxxxxxxxxxxx>, x86@xxxxxxxxxx, kernel-janitors@xxxxxxxxxxxxxxx, Chris Wright <chrisw@xxxxxxxxxxxx>, virtualization@xxxxxxxxxxxxxx, Ingo Molnar <mingo@xxxxxxxxxx>, Tejun Heo <tj@xxxxxxxxxx>, Thomas Gleixner <tglx@xxxxxxxxxxxxx>, Ian Campbell <ian.campbell@xxxxxxxxxx>
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 On 09/14/2010 11:20 AM, H. Peter Anvin wrote:
> On 09/14/2010 10:49 AM, Jeremy Fitzhardinge wrote:
>>  On 09/13/2010 10:18 PM, matt mooney wrote:
>>> Unnecessary cast from void* in assignment.
>> Not very keen on this.  The cast may not be strictly required, but it
>> does document what's going on there.
> But unnecessary casts are problematic in that if the type changes, they
> can hide a real bug in the future.

If page->private changes in a way which invalidates this kind of usage,
I would hope it would also change name to something more descriptive of
its new role.  If it changes type but is broadly compatible with this
usage (say, void * -> unsigned long), then the cast won't matter, or
will become a requirement.

I'm more concerned about "user_ptr" changing type, in which case the
cast will highlight the problem.

In general, the casts from these kinds of generic/polymorphic fields let
you easily eyeball how they're being used, without having to work out
what the type of the other side is.


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