2015-07-09 13:34:50

by Alexander Gordeev

Subject: [PATCH] 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" <[email protected]>
Suggested-by: "Paul E. McKenney" <[email protected]>
Signed-off-by: Alexander Gordeev <[email protected]>
---
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 852b810..543ba72 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -451,7 +451,9 @@ 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. */
--
1.8.3.1

2015-07-09 17:01:29

by Paul E. McKenney

Subject: Re: [PATCH] rcu: Shut up bogus gcc array bounds warning

On Thu, Jul 09, 2015 at 03:34:23PM +0200, Alexander Gordeev wrote:
> 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" <[email protected]>
> Suggested-by: "Paul E. McKenney" <[email protected]>
> Signed-off-by: Alexander Gordeev <[email protected]>

Queued, thank you!

Thanx, Paul

> ---
> 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 852b810..543ba72 100644
> --- a/kernel/rcu/tree.h
> +++ b/kernel/rcu/tree.h
> @@ -451,7 +451,9 @@ 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. */
> --
> 1.8.3.1
>