On Wed, 13 Feb 2008 16:14:04 -0800 "Paul E. McKenney" <[EMAIL PROTECTED]> wrote:
> On Wed, Feb 13, 2008 at 03:51:58PM -0800, Stephen Hemminger wrote: > > On Wed, 13 Feb 2008 15:37:44 -0800 > > "Paul E. McKenney" <[EMAIL PROTECTED]> wrote: > > > On Wed, Feb 13, 2008 at 02:42:33PM -0800, Stephen Hemminger wrote: > > > > On Wed, 13 Feb 2008 14:41:34 -0800 > > > > "Paul E. McKenney" <[EMAIL PROTECTED]> wrote: > > > > > > > > > On Wed, Feb 13, 2008 at 02:35:37PM -0800, Stephen Hemminger wrote: > > > > > > On Wed, 13 Feb 2008 14:00:24 -0800 > > > > > > "Paul E. McKenney" <[EMAIL PROTECTED]> wrote: > > > > > > > > > > > > > Hello! > > > > > > > > > > > > > > This is an updated version of the patch posted last November: > > > > > > > > > > > > > > > > > > > > > http://archives.free.net.ph/message/20071201.003721.cd6ff17c.en.html > > > > > > > > > > > > > > This new version permits arguments with side effects, for example: > > > > > > > > > > > > > > rcu_assign_pointer(global_p, p++); > > > > > > > > > > > > > > and also verifies that the arguments are pointers, while still > > > > > > > avoiding > > > > > > > the unnecessary memory barrier when assigning NULL to a pointer. > > > > > > > This memory-barrier avoidance means that rcu_assign_pointer() is > > > > > > > now only > > > > > > > permitted for pointers (not array indexes), and so this version > > > > > > > emits a > > > > > > > compiler warning if the first argument is not a pointer. I built > > > > > > > a "make > > > > > > > allyesconfig" version on an x86 system, and received no such > > > > > > > warnings. > > > > > > > If RCU is ever applied to array indexes, then the second patch in > > > > > > > this > > > > > > > series should be applied, and the resulting rcu_assign_index() be > > > > > > > used. > > > > > > > > > > > > > > Given the rather surprising history of subtlely broken > > > > > > > implementations of > > > > > > > rcu_assign_pointer(), I took the precaution of generating a full > > > > > > > set of > > > > > > > test cases and verified that memory barriers and compiler > > > > > > > warnings were > > > > > > > emitted when required. I guess it is the simple things that get > > > > > > > you... > > > > > > > > > > > > > > Signed-off-by: Paul E. McKenney <[EMAIL PROTECTED]> > > > > > > > --- > > > > > > > > > > > > > > rcupdate.h | 16 ++++++++++++---- > > > > > > > 1 file changed, 12 insertions(+), 4 deletions(-) > > > > > > > > > > > > > > diff -urpNa -X dontdiff linux-2.6.24/include/linux/rcupdate.h > > > > > > > linux-2.6.24-rap/include/linux/rcupdate.h > > > > > > > --- linux-2.6.24/include/linux/rcupdate.h 2008-01-24 > > > > > > > 14:58:37.000000000 -0800 > > > > > > > +++ linux-2.6.24-rap/include/linux/rcupdate.h 2008-02-13 > > > > > > > 13:36:47.000000000 -0800 > > > > > > > @@ -270,12 +270,20 @@ extern struct lockdep_map rcu_lock_map; > > > > > > > * structure after the pointer assignment. More importantly, > > > > > > > this > > > > > > > * call documents which pointers will be dereferenced by RCU > > > > > > > read-side > > > > > > > * code. > > > > > > > + * > > > > > > > + * Throws a compiler warning for non-pointer arguments. > > > > > > > + * > > > > > > > + * Does not insert a memory barrier for a NULL pointer. > > > > > > > */ > > > > > > > > > > > > > > -#define rcu_assign_pointer(p, v) ({ \ > > > > > > > - smp_wmb(); \ > > > > > > > - (p) = (v); \ > > > > > > > - }) > > > > > > > +#define rcu_assign_pointer(p, v) \ > > > > > > > + ({ \ > > > > > > > + typeof(*p) *_________p1 = (v); \ > > > > > > > + \ > > > > > > > + if (!__builtin_constant_p(v) || (_________p1 != NULL)) \ > > > > > > > + smp_wmb(); \ > > > > > > > + (p) = _________p1; \ > > > > > > > + }) > > > > > > > > > > > > > > /** > > > > > > > * synchronize_sched - block until all CPUs have exited any > > > > > > > non-preemptive > > > > > > > > > > > > Will this still work if p is unsigned long? > > > > > > > > > > Hello, Steve, > > > > > > > > > > If p is unsigned long, then use rcu_assign_index() from the next > > > > > patch in > > > > > the set. Looks like Andrew has applied it to -mm -- so please make > > > > > sure > > > > > that he is aware if you do use it. > > > > > > > > Make sure fib_trie still works and doesn't get warnings. > > > > > > Ah. It does take a bit to get fib_trie into one's build -- allyesconfig > > > doesn't cut it. Please accept my apologies for my confusion!!! > > > > > > Once fib_trie is configured, I do indeed get: > > > > > > net/ipv4/fib_trie.c: In function ‘node_set_parent’: > > > net/ipv4/fib_trie.c:182: warning: comparison between pointer and > > > integer > > > > > > So, given that node->parent is an unsigned long, I changed > > > node_set_parent() > > > to the following: > > > > > > static inline void node_set_parent(struct node *node, struct tnode *ptr) > > > { > > > rcu_assign_index(node->parent, (unsigned long)ptr | NODE_TYPE(node)); > > > } > > > > > > This removes the warnings. I am a little ambivalent about this, as > > > this is really a pointer in disguise rather than an array index, but > > > patch below. I suppose that another option would be to make node->parent > > > be a void* and provide appropriate accessor functions/macros. > > > > > > Thoughts? > > > > > > Signed-off-by: Paul E. McKenney <[EMAIL PROTECTED]> > > > > Maybe cast both sides to void * in this case: > > > > static inline void node_set_parent(struct node *node, struct tnode *ptr) > > { > > rcu_assign_pointer((void *) node->parent, (void *)((unsigned long)ptr | > > NODE_TYPE(node))); > > } > > That gets me the following: > > net/ipv4/fib_trie.c: In function ‘node_set_parent’: > net/ipv4/fib_trie.c:182: error: invalid lvalue in assignment > > However, as with much in computing, an extra level of indirection fixes > things. Your call as to whether or not the cure is preferable to the > disease. ;-) > > Signed-off-by: Paul E. McKenney <[EMAIL PROTECTED]> > --- > > fib_trie.c | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff -urpNa -X dontdiff linux-2.6.25-rc1/net/ipv4/fib_trie.c > linux-2.6.25-rc1-fib_trie-warn.compile/net/ipv4/fib_trie.c > --- linux-2.6.25-rc1/net/ipv4/fib_trie.c 2008-02-13 14:38:12.000000000 > -0800 > +++ linux-2.6.25-rc1-fib_trie-warn.compile/net/ipv4/fib_trie.c > 2008-02-13 16:10:07.000000000 -0800 > @@ -179,8 +179,8 @@ static inline struct tnode *node_parent_ > > static inline void node_set_parent(struct node *node, struct tnode *ptr) > { > - rcu_assign_pointer(node->parent, > - (unsigned long)ptr | NODE_TYPE(node)); > + rcu_assign_pointer((*(void **)&node->parent), > + (void *)((unsigned long)ptr | NODE_TYPE(node))); > } That is heading towards ugly... Maybe not using the macro at all (for this case) would be best: static inline void node_set_parent(struct node *node, struct tnode *ptr) { smp_wmb(); node->parent = (unsigned long)ptr | NODE_TYPE(node); } -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
