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



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