[PATCH v2] cpumask: add cpumask_var_t documentation

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

cpumask_var_t has one nortable difference against cpumask_t.
This patch adds the explanation.

Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
---
 include/linux/cpumask.h |   14 ++++++++++++++
 1 files changed, 14 insertions(+), 0 deletions(-)

diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
index 1e40dd0..471c98a 100644
--- a/include/linux/cpumask.h
+++ b/include/linux/cpumask.h
@@ -617,6 +617,20 @@ static inline size_t cpumask_size(void)
  *	  ... use 'tmpmask' like a normal struct cpumask * ...
  *
  *	free_cpumask_var(tmpmask);
+ *
+ *
+ * However, one notable exception is there. cpumask_var_t is allocated
+ * only nr_cpu_ids bits (in the other hand, real cpumask_t always has
+ * NR_CPUS bits). therefore You don't have to dereference cpumask_var_t.
+ *
+ *	cpumask_var_t tmpmask;
+ *	if (!alloc_cpumask_var(&tmpmask, GFP_KERNEL))
+ *		return -ENOMEM;
+ *
+ *	var = *tmpmask;
+ *
+ * This code makes NR_CPUS length memcopy and bring memroy corruption.
+ * You have to use cpumask_copy() instead.
  */
 #ifdef CONFIG_CPUMASK_OFFSTACK
 typedef struct cpumask *cpumask_var_t;
-- 
1.7.3.1