Re: [PATCH 3/4] fib_trie: print statistics for multiple tables

Paul E. McKenney
2008-02-13T22:26:58Z

On Tue, Feb 12, 2008 at 06:35:21PM -0800, Andrew Morton wrote:
> On Tue, 12 Feb 2008 16:50:45 -0800 Stephen Hemminger <shemminger@vyatta.com> wrote:
>
> > +/**
> > + * hlist_for_each_entry_continue_rcu - iterate over rcu hlist after current point
> > + * @tpos: the type * to use as a loop cursor.
> > + * @pos: the &struct hlist_node to use as a loop cursor.
> > + * @member: the name of the hlist_node within the struct.
> > + */
> > +#define hlist_for_each_entry_continue_rcu(tpos, pos, member) \
> > + for (pos = (pos)->next; \
> > + rcu_dereference(pos) && ({ prefetch(pos->next); 1;}) && \
> > + ({ tpos = hlist_entry(pos, typeof(*tpos), member); 1;}); \
> > + pos = pos->next)
>
> Is the compiler allowed to look at a term such as
>
> ({ prefetch(pos->next); 1;})
>
> and, when it is used as a truth value, say "hey, that's always true" and
> then elide the call to prefetch()? We've no way of telling because this
> remains gcc-specific territory, afaik.

The prefetch() definitions I found are "asm volatile". So, as I
understand it, the compiler is not supposed to remove it, just as it
would not be permitted to remove something that could have a side effect.

> (cc Paul for rcu stuff)

Given my track record with simple functions of late :-/ I will beat this
one up a bit...

Thanx, Paul