Re: [PATCH tip/core/rcu 4/6] rcu: Silence compiler array out-of-bounds false positive

[Josh Triplett <josh@joshtriplett.org>]

On Sat, Jan 05, 2013 at 09:09:36AM -0800, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> 
> 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>
> ---
>  kernel/rcutree.c |    4 ++++
>  1 files changed, 4 insertions(+), 0 deletions(-)
> 
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index d145796..e0d9815 100644
> --- 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");

Why not write this as BUG_ON(rcu_num_lvls > RCU_NUM_LVLS)?  Given that
the condition in question can never happen, you don't really need an
explanatory message.

I do find it surprising, though, that the compiler can't figure this one
out, given that rcu_num_lvls gets initialized right before this in the
same file (and likely inlined into the same function).  I wonder if it
thought some other code might change it unexpectedly, since rcu_num_lvls
doesn't get declared as static?  Unfortunately, the loop macros in
rcutree.h make it difficult to make rcu_num_lvls static, but as far as I
can tell only one use of those macros ever gets expanded outside of
rcutree.c: the one in rcutree_trace.c.  If you compile out tracing, and
declare rcu_num_lvls static, does the warning go away?

- Josh Triplett