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
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