rcu: Silence compiler array out-of-bounds false positive

It turns out that gcc 4.8 warns on array indexes being out of bounds
unless it can prove otherwise.  It gives this warning on some RCU
initialization code.  Because this is far from any fastpath, add
an explicit check for array bounds and panic if so.  This gives the
compiler enough information to figure out that the array index is never
out of bounds.

However, if a similar false positive occurs on a fastpath, it will
probably be necessary to tell the compiler to keep its array-index
anxieties to itself.  ;-)

Markus Trippelsdorf <markus@trippelsdorf.de>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index d145796bd61f7f2ca246902a92e1d5026ec3ef51..e0d98157fbea872a5ac89bcae0f1d93a645f5f8f 100644 (file)
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -2938,6 +2938,10 @@ static void __init rcu_init_one(struct rcu_state *rsp,
 
        BUILD_BUG_ON(MAX_RCU_LVLS > ARRAY_SIZE(buf));  /* Fix buf[] init! */
 
+       /* Silence gcc 4.8 warning about array index out of range. */
+       if (rcu_num_lvls > RCU_NUM_LVLS)
+               panic("rcu_init_one: rcu_num_lvls overflow");
+
        /* Initialize the level-tracking arrays. */
 
        for (i = 0; i < rcu_num_lvls; i++)