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Subject[RFC PATCH 2/2] kernel/sched.c: VLA in middle of struct
The semantics for variable-length arrays __in the middle of structs__
are quite muddy, and a case in sched.c presents an interesting case,
as the preceding code comment indicates:

/*
* The cpus mask in sched_group and sched_domain hangs off
the end. * FIXME: use cpumask_var_t or dynamic percpu alloc
to avoid * wasting space for nr_cpu_ids < CONFIG_NR_CPUS. */
struct static_sched_group {
struct sched_group sg; DECLARE_BITMAP(cpus,
CONFIG_NR_CPUS);
};

struct static_sched_domain {
struct sched_domain sd; DECLARE_BITMAP(span,
CONFIG_NR_CPUS);
};

Both sched_group and sched_domain have the following trailing struct
member:

unsigned long cpumask[];

So this change is intended largely to spawn a discussion, because
I'm not sure this VLA-in-middle-of-struct behavior is guaranteed to
always behave as expected?

Maybe a C expert can say whether cpumask[0] is better than cpumask[],
or have other comments?

Obviously not to be applied...

NOT-Signed-off-by: Jeff Garzik <jgarzik@redhat.com>
----
kernel/sched.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/sched.c b/kernel/sched.c
index 3d6b183..8056d74 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -7757,13 +7757,13 @@ int sched_smt_power_savings = 0, sched_mc_power_savings = 0;
* for nr_cpu_ids < CONFIG_NR_CPUS.
*/
struct static_sched_group {
- struct sched_group sg;
DECLARE_BITMAP(cpus, CONFIG_NR_CPUS);
+ struct sched_group sg;
};

struct static_sched_domain {
- struct sched_domain sd;
DECLARE_BITMAP(span, CONFIG_NR_CPUS);
+ struct sched_domain sd;
};

/*

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