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[PATCH tip/core/rcu 11/12] rcu: Shut up bogus gcc array bounds warning

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From: Alexander Gordeev <agordeev@redhat.com>

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>
---
 kernel/rcu/tree.h | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 581f8d3c5b28..faee5242d6ff 100644
--- 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. */
-- 
1.8.1.5

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[PATCH tip/core/rcu 0/12] Tree geometry-initialization  simplifications for 4.3 "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-07-18 00:40 +0200
  [PATCH tip/core/rcu 03/12] rcu: Remove superfluous local variable in rcu_init_geometry() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-07-18 00:40 +0200
  [PATCH tip/core/rcu 08/12] rcu: Remove unnecessary fields from rcu_state structure "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-07-18 00:40 +0200
  [PATCH tip/core/rcu 12/12] rcu: Reset rcu_fanout_leaf if out of bounds "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-07-18 00:40 +0200
  [PATCH tip/core/rcu 11/12] rcu: Shut up bogus gcc array bounds warning "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-07-18 00:40 +0200
  [PATCH tip/core/rcu 10/12] rcu: Simplify arithmetic to calculate number of RCU nodes "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-07-18 00:40 +0200
  [PATCH tip/core/rcu 05/12] rcu: Simplify rcu_init_geometry() capacity arithmetics "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-07-18 00:40 +0200
  [PATCH tip/core/rcu 09/12] rcu: Limit count of static data to the number of RCU levels "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-07-18 00:40 +0200
  [PATCH tip/core/rcu 01/12] rcu: Provide more diagnostics for stalled GP kthread "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-07-18 00:40 +0200
    [PATCH tip/core/rcu 04/12] rcu: Cleanup rcu_init_geometry() code and arithmetics "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-07-18 00:40 +0200
    [PATCH tip/core/rcu 02/12] rcu: Panic if RCU tree can not accommodate all CPUs "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-07-18 00:40 +0200
    [PATCH tip/core/rcu 07/12] rcu: Limit rcu_capacity[] size to RCU_NUM_LVLS items "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-07-18 00:40 +0200
    [PATCH tip/core/rcu 06/12] rcu: Limit rcu_state::levelcnt[] to RCU_NUM_LVLS items "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-07-18 00:40 +0200

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