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

[Josh Triplett <josh@joshtriplett.org>]

On Mon, Jan 07, 2013 at 07:08:55PM +0100, Markus Trippelsdorf wrote:
> On 2013.01.07 at 09:16 -0800, Paul E. McKenney wrote:
> > 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");
> > 
> > > 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.
> 
> Yes, declaring rcu_num_lvls static would fix the issue. See the
> following gcc bug: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55529

Seems potentially worth restructuring the code a bit to fold in the one
rcutree_trace.c bit that needs it and then making rcu_num_lvls static
and internal to rcutree.c.

- Josh Triplett