sched: properly define the sched_group::cpumask and sched_domain::span fields

Properly document the variable-size structure tricks we are doing
wrt. struct sched_group and sched_domain, and use the field[0] GCC
extension instead of defining a vla array.

Dont use unions for this, as pointed out by Linus.

[ Impact: cleanup, un-confuse Sparse and LLVM ]

Reported-by: Jeff Garzik <jeff@garzik.org>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
LKML-Reference: <alpine.LFD.2.01.0905180850110.3301@localhost.localdomain>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 22 insertions, 3 deletions

diff --git a/include/linux/sched.h b/include/linux/sched.h
index de7b3b217772..dbb1043e8656 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -839,7 +839,17 @@ struct sched_group {
          */
         u32 reciprocal_cpu_power;
 
-        unsigned long cpumask[];
+        /*
+         * The CPUs this group covers.
+         *
+         * NOTE: this field is variable length. (Allocated dynamically
+         * by attaching extra space to the end of the structure,
+         * depending on how many CPUs the kernel has booted up with)
+         *
+         * It is also be embedded into static data structures at build
+         * time. (See 'struct static_sched_group' in kernel/sched.c)
+         */
+        unsigned long cpumask[0];
 };
 
 static inline struct cpumask *sched_group_cpus(struct sched_group *sg)
@@ -925,8 +935,17 @@ struct sched_domain {
         char *name;
 #endif
 
-        /* span of all CPUs in this domain */
-        unsigned long span[];
+        /*
+         * Span of all CPUs in this domain.
+         *
+         * NOTE: this field is variable length. (Allocated dynamically
+         * by attaching extra space to the end of the structure,
+         * depending on how many CPUs the kernel has booted up with)
+         *
+         * It is also be embedded into static data structures at build
+         * time. (See 'struct static_sched_domain' in kernel/sched.c)
+         */
+        unsigned long span[0];
 };
 
 static inline struct cpumask *sched_domain_span(struct sched_domain *sd)