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-rw-r--r--include/linux/types.h16
1 files changed, 15 insertions, 1 deletions
diff --git a/include/linux/types.h b/include/linux/types.h
index c314989d9158..70d8500bddf1 100644
--- a/include/linux/types.h
+++ b/include/linux/types.h
@@ -205,11 +205,25 @@ struct ustat {
* struct callback_head - callback structure for use with RCU and task_work
* @next: next update requests in a list
* @func: actual update function to call after the grace period.
+ *
+ * The struct is aligned to size of pointer. On most architectures it happens
+ * naturally due ABI requirements, but some architectures (like CRIS) have
+ * weird ABI and we need to ask it explicitly.
+ *
+ * The alignment is required to guarantee that bits 0 and 1 of @next will be
+ * clear under normal conditions -- as long as we use call_rcu(),
+ * call_rcu_bh(), call_rcu_sched(), or call_srcu() to queue callback.
+ *
+ * This guarantee is important for few reasons:
+ * - future call_rcu_lazy() will make use of lower bits in the pointer;
+ * - the structure shares storage spacer in struct page with @compound_head,
+ * which encode PageTail() in bit 0. The guarantee is needed to avoid
+ * false-positive PageTail().
*/
struct callback_head {
struct callback_head *next;
void (*func)(struct callback_head *head);
-};
+} __attribute__((aligned(sizeof(void *))));
#define rcu_head callback_head
typedef void (*rcu_callback_t)(struct rcu_head *head);