Subject: Re: net/ipv4/fib_trie.c - compile error (Re: 2.6.23-rc3-mm1)

From: "Paul E. McKenney" <[email-masked]>
Date: Mon, 27 Aug 2007 09:23:18 -0700

On Mon, Aug 27, 2007 at 08:36:35AM +0200, Jarek Poplawski wrote:
> On 22-08-2007 19:03, Paul E. McKenney wrote:
> > On Wed, Aug 22, 2007 at 05:41:11PM +0200, Adrian Bunk wrote:
> >> On Wed, Aug 22, 2007 at 05:30:13PM +0200, Gabriel C wrote:
> >>> Got it with a randconfig ( http://194.231.229.228/kernel/mm/2.6.23-rc3-mm1/r/randconfig-8 )
> >>>
> >>> ...
> >>>
> >>> net/ipv4/fib_trie.c: In function 'trie_rebalance':
> >>> net/ipv4/fib_trie.c:969: error: lvalue required as unary '&' operand
> >>> net/ipv4/fib_trie.c:971: error: lvalue required as unary '&' operand
> >>> net/ipv4/fib_trie.c:977: error: lvalue required as unary '&' operand
> >>> net/ipv4/fib_trie.c:980: error: lvalue required as unary '&' operand
> >>> ...
> >> Side effect of the git-net removal, temporarily removing 
> >> immunize-rcu_dereference-against-crazy-compiler-writers.patch should 
> >> work around it.
> > 
> > Alternatively, the following one-line patch to net/ipv4/fib_trie.c could
> > be used.
> > 
> > Signed-off-by: Paul E. McKenney <[email-masked]>
> > ---
> > 
> >  fib_trie.c |    2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff -urpNa -X dontdiff linux-2.6.23-rc3-mm1/net/ipv4/fib_trie.c linux-2.6.23-rc3-mm1.compile/net/ipv4/fib_trie.c
> > --- linux-2.6.23-rc3-mm1/net/ipv4/fib_trie.c	2007-08-22 09:20:33.000000000 -0700
> > +++ linux-2.6.23-rc3-mm1.compile/net/ipv4/fib_trie.c	2007-08-22 09:47:33.000000000 -0700
> > @@ -94,7 +94,7 @@ typedef unsigned int t_key;
> >  #define T_LEAF  1
> >  #define NODE_TYPE_MASK	0x1UL
> >  #define NODE_PARENT(node) \
> > -	((struct tnode *)rcu_dereference(((node)->parent & ~NODE_TYPE_MASK)))
> > +	((struct tnode *)(rcu_dereference((node)->parent) & ~NODE_TYPE_MASK))
> ...
> 
> After first reading of this thread I've had an impression it's about
> compiler's behavior, but now it seems to me this patch is not an
> alternative, but a 'must be' and only proper way of calling
> rcu_dereference (with a variable instead of an expression)? Am I
> right?

Yes, rcu_dereference() does indeed need to be invoked on a lvalue.

							Thanx, Paul