linux-kernel - Re: [PATCH] rcu: Shut up bogus gcc array bounds warning

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Message-ID: <20150709170110.GB3717@linux.vnet.ibm.com>
Date:	Thu, 9 Jul 2015 10:01:10 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Alexander Gordeev <agordeev@...hat.com>
Cc:	linux-kernel@...r.kernel.org
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" <paulmck@...ux.vnet.ibm.com>
> Suggested-by: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> Signed-off-by: Alexander Gordeev <agordeev@...hat.com>

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
> 

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