linux-kernel - Re: [RFC PATCH 2/2] kernel/sched.c: VLA in middle of struct

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Message-Id: <200905122304.52395.rusty@rustcorp.com.au>
Date:	Tue, 12 May 2009 23:04:51 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Jeff Garzik <jeff@...zik.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mike Travis <travis@....com>,
	LKML <linux-kernel@...r.kernel.org>, viro@...iv.linux.org.uk,
	Andrew Morton <akpm@...ux-foundation.org>, roland@...hat.com
Subject: Re: [RFC PATCH 2/2] kernel/sched.c: VLA in middle of struct

On Mon, 11 May 2009 12:39:54 am Jeff Garzik wrote:
> On Sun, May 10, 2009 at 06:19:40PM +0930, Rusty Russell wrote:
> > Yeah, it's kinda nasty.  Generally, sched_group is dynamically allocated,
> > so we just allocate sizeof(struct sched_group) + size of nr_cpu_ids bits.
> >
> > These ones are static, and it was easier to put this hack in than make
> > them dynamic.  There's nothing wrong with it, until we really want
> > NR_CPUS == bignum, or we want to get rid of NR_CPUS altogether for
> > CONFIG_CPUMASKS_OFFSTACK (which would be very clean, but not clearly
> > worthwhile).
>
> Nothing wrong with it, except
>
> - C99 only defines variable-length automatic arrays
> - VLA in the middle of a struct are difficult to optimize
> - gcc's VLA handling WILL change, as gcc docs state
> - other compilers -- and sparse -- puke all over VLAs, making
>   static analysis impossible for all code with this weirdism

Jeff, you seem confused.  In my copy of the standard, you'd know this is called 
a "flexible array member"; it's not a variable length array.  The only GCC 
specific issue I can find here is that you're not normally allowed to embed 
structs with them in another struct (according to the gcc docs; I can't 
actually find this clearly stated in the standard).

> > But more importantly, my comment is obviously unclear, since your patch
> > shows you didn't understand the purpose of the field: The cpus bitmap
> > *is* the sg-
> >
> > >cpumask[] array.
>
> I guess you missed the
> (1) "this patch is only intended to spark discussion",
> (2) a reference to the comment, and
> (3) "NOT-signed-off-by" portions of my email.

Terribly sorry, I was too polite.  Your patch was so broken it didn't make any 
sense.  At all.

Anyway, since [] is C99, I thought it preferable to [0] which is a gcc 
extension.  However, if C99 is really so braindead as to disallow this fairly 
standard trick, so I'm happy to go with the gcc extension.[1]

Thanks,
Rusty.
[1] Well, not happy.  But y'know...
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