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

Paul E. McKenney
2013-01-07T17:17:30Z

On Mon, Jan 07, 2013 at 07:50:02AM -0800, Josh Triplett wrote:
> 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.

Good point, will do!

> 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?

I found it quite surprising also, hence the "array-index anxieties" above.

I added Marcus on CC for his thoughts on this.

Thanx, Paul