[PATCH] cpumask: add cpumask_var_t documentation

[KOSAKI Motohiro]

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

Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
---
 include/linux/cpumask.h |    6 ++++++
 1 files changed, 6 insertions(+), 0 deletions(-)

diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
index a3ff24f..3d09505 100644
--- a/include/linux/cpumask.h
+++ b/include/linux/cpumask.h
@@ -617,6 +617,12 @@ 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 cpumask_var_t can't use '=' operator.
+ * It makes NR_CPUS size 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



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