Messages in this thread | | | Subject | Re: [PATCH tip/core/rcu 4/6] rcu: Silence compiler array out-of-bounds false positive | From | Steven Rostedt <> | Date | Mon, 07 Jan 2013 17:18:37 -0500 |
| |
On Mon, 2013-01-07 at 09:19 -0800, Paul E. McKenney wrote: > On Mon, Jan 07, 2013 at 09:16:02AM -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"); > > > > > > 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! > > Ah, wait, BUG_ON() sometimes compiles to nothingness: > > #ifndef HAVE_ARCH_BUG_ON > #define BUG_ON(condition) do { if (condition) ; } while(0) > #endif > > So I do need the explicit "if". :-(
Bah, those archs shouldn't be bothered with. If they don't want to bug, then that's there problem :-)
Lots of places in the kernel have BUG_ON() where they require it to panic.
-- Steve
| |