On 2012.11.29 at 10:10 -0800, Paul E. McKenney wrote:
> On Thu, Nov 29, 2012 at 06:43:58PM +0100, Markus Trippelsdorf wrote:
> > 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?
>
> Good question. Are you saying that if the compiler cannot prove that
> the index is in bounds, it is going to throw a warning?
Yes, it does seem to be the case. See also my gcc bug report (closed as
invalid): http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55529
> If that is the case, perhaps telling the compiler to cool it via the
> command line would be best.
> Or is this really one of a very few places in the kernel where the
> compiler is complaining?
Yes. With my (admittedly minimal) config this is only place.
--
Markus
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