Messages in this thread | | | Date | Mon, 7 Jan 2013 09:16:02 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH tip/core/rcu 4/6] rcu: Silence compiler array out-of-bounds false positive |
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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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