rcu: Shut up bogus gcc array bounds warning

Because gcc does not realize a loop would not be entered ever
(i.e. in case of rcu_num_lvls == 1):

  for (i = 1; i < rcu_num_lvls; i++)
	  rsp->level[i] = rsp->level[i - 1] + levelcnt[i - 1];

some compiler (pre- 5.x?) versions give a bogus warning:

  kernel/rcu/tree.c: In function ‘rcu_init_one.isra.55’:
  kernel/rcu/tree.c:4108:13: warning: array subscript is above array bounds [-Warray-bounds]
     rsp->level[i] = rsp->level[i - 1] + rsp->levelcnt[i - 1];
               ^
Fix that warning by adding an extra item to rcu_state::level[]
array. Once the bogus warning is fixed in gcc and kernel drops
support of older versions, the dummy item may be removed from
the array.

Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Suggested-by: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 581f8d3..faee524 100644 (file)
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -443,7 +443,9 @@ do {                                                                        \
  */
 struct rcu_state {
        struct rcu_node node[NUM_RCU_NODES];    /* Hierarchy. */
-       struct rcu_node *level[RCU_NUM_LVLS];   /* Hierarchy levels. */
+       struct rcu_node *level[RCU_NUM_LVLS + 1];
+                                               /* Hierarchy levels (+1 to */
+                                               /*  shut bogus gcc warning) */
        u8 flavor_mask;                         /* bit in flavor mask. */
        struct rcu_data __percpu *rda;          /* pointer of percu rcu_data. */
        void (*call)(struct rcu_head *head,     /* call_rcu() flavor. */