[PATCH] rcu: force alignment on struct callback_head/rcu_head

[Kirill A. Shutemov]

This patch makes struct callback_head 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().

False postive PageTail() caused crash on crisv32[1]. It happend due
misaligned task_struct->rcu, which was byte-aligned.

[1] http://lkml.kernel.org/r/55FAEA67.9000102@xxxxxxxxxxxx

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Reported-by: Guenter Roeck <linux@xxxxxxxxxxxx>
---
 include/linux/types.h | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

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);
-- 
 Kirill A. Shutemov
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