Re: kernel/rcutree.c:2850:13: warning: array subscript is above array bounds

[Markus Trippelsdorf <markus@trippelsdorf.de>]

On 2012.11.29 at 09:02 -0800, Paul E. McKenney wrote:
> On Thu, Nov 29, 2012 at 02:47:52PM +0100, Markus Trippelsdorf wrote:
> > With gcc-4.8 I get:
> > 
> >   CC      kernel/rcutree.o
> >   kernel/rcutree.c: In function ‘rcu_init_one’:
> >   kernel/rcutree.c:2850:13: warning: array subscript is above array bounds [-Warray-bounds]
> >      rsp->level[i] = rsp->level[i - 1] + rsp->levelcnt[i - 1];
> >                   ^
> > 2849    for (i = 1; i < rcu_num_lvls; i++)
> > 2850           rsp->level[i] = rsp->level[i - 1] + rsp->levelcnt[i - 1];
> > 
> > At first I thought that the warning was bogus, but rcu_num_lvls isn't static
> > and gets modified prior to the for loop.
> 
> You are quite correct that rcu_num_lvls does get modified, but there
> are checks in rcu_init_geometry() to ensure that it does not increase:
> 
> 	/*
> 	 * The boot-time rcu_fanout_leaf parameter is only permitted
> 	 * to increase the leaf-level fanout, not decrease it.  Of course,
> 	 * the leaf-level fanout cannot exceed the number of bits in
> 	 * the rcu_node masks.  Finally, the tree must be able to accommodate
> 	 * the configured number of CPUs.  Complain and fall back to the
> 	 * compile-time values if these limits are exceeded.
> 	 */
> 	if (rcu_fanout_leaf < CONFIG_RCU_FANOUT_LEAF ||
> 	    rcu_fanout_leaf > sizeof(unsigned long) * 8 ||
> 	    n > rcu_capacity[MAX_RCU_LVLS]) {
> 		WARN_ON(1);
> 		return;
> 	}
> 
> The value of rcu_num_lvls starts out at RCU_NUM_LVLS, the same as
> the dimension of the ->level[] array.  The loop goes only to one less
> than rcu_num_lvls, as needed, and rcu_num_lvls is never greater than
> RCU_NUM_LVLS, so this should be safe.
> 
> So what am I missing here?

rcu_num_lvls does get modified in rcu_init_geometry:

2942         /* Calculate the number of rcu_nodes at each level of the tree. */
2943         for (i = 1; i <= MAX_RCU_LVLS; i++)
2944                 if (n <= rcu_capacity[i]) {
2945                         for (j = 0; j <= i; j++)
2946                                 num_rcu_lvl[j] =
2947                                         DIV_ROUND_UP(n, rcu_capacity[i - j]);
2948                         rcu_num_lvls = i;

And rcu_init_geometry gets called before rcu_init_one, so the compiler assumes
the worst and issues a warning.
So, in your opinion, what would be the best way to silence this warning?

-- 
Markus