Re: [GIT PULL] scheduler fixes

Ingo Molnar
2009-05-18T16:50:58Z

* Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Mon, 18 May 2009, Ingo Molnar wrote:
> >
> > Rusty Russell (1):
> > sched: avoid flexible array member inside struct (gcc extension)
>
> I'm not pulling this one either.
>
> It makes no sense what-so-ever. It's uglier code, so calling it a
> cleanup is just wrong.

hm - i've Cc:-ed Jeff & Viro. The background is that Sparse and LLVM
barfed on the current construct and Al strongly advocated this
solution, see:

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

See that particular reply below.

Ingo

----- Forwarded message from Al Viro <viro@ZenIV.linux.org.uk> -----

Date: Tue, 12 May 2009 15:03:44 +0100
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Rusty Russell <rusty@rustcorp.com.au>
Subject: Re: [RFC PATCH 2/2] kernel/sched.c: VLA in middle of struct
Cc: Jeff Garzik <jeff@garzik.org>, Ingo Molnar <mingo@elte.hu>,
Peter Zijlstra <a.p.zijlstra@chello.nl>,
Mike Travis <travis@sgi.com>, LKML <linux-kernel@vger.kernel.org>,
Andrew Morton <akpm@linux-foundation.org>, roland@redhat.com

On Tue, May 12, 2009 at 11:04:51PM +0930, Rusty Russell wrote:
> 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).

6.7.2.1p2. It's a separate issue from revolting gcc extension that *do*
allow VLA-in-the-middle-of-struct. And I mean real VLA, not flex array
member :-/

> 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]

No. There's a standard way to do that in C99; you can put a struct with
that thing into a union. So correct way to force enought storage for
such an object is

union {
struct has_flex_array_member foo;
char [how_much_space_do_I_want];
} bar;

Unions with overlapping members are fine. Structures are not.

----- End forwarded message -----