On Fri, Oct 4, 2013 at 11:46 AM, Oleg Nesterov <o...@redhat.com> wrote:
> Add the new struct rcu_sync_ops which holds sync/call methods, and
> turn the function pointers in rcu_sync_struct into the single pointer
> to struct rcu_sync_ops.
>
> +struct rcu_sync_ops {
> + void (*sync)(void);
> + void (*call)(struct rcu_head *, void (*)(struct rcu_head *));
> +};
> +
> struct rcu_sync_struct {
> int gp_state;
> int gp_count;
> @@ -12,43 +17,9 @@ struct rcu_sync_struct {
> int cb_state;
> struct rcu_head cb_head;
>
> - void (*sync)(void);
> - void (*call)(struct rcu_head *, void (*)(struct rcu_head *));
> + struct rcu_sync_ops *ops;
Ugh.
This interface pretty much guarantees that a compiler can never do
anything clever, like know that "hey, you used a static initializer on
this thing, and the fields are const, so now know statically what the
functions are, and I can just turn the indirect jumps into direct
jumps".
I'm not sure gcc is actually that clever, but by making it this kind
of ops pointer, I *guarantee* that gcc can never do it.
How about you make the rule be:
- get rid of the stupid "type" enum index thing
- get rid of the "init" thing that sets pointers in the dynamic data
structures. Get rid of the pointer too.
- instead, use a "static const" type descriptor for each type (it
approaches being your "rcu_sync_ops" structure). Pass this in as an
argument to all the functions (use a #define per type or something, so
that users don't need to do this by hand)
- now every single user passes in that type descriptor.
- together with using a few inline functions, suddenly the "indirect"
jumps through this type descriptor end up actually being nice direct
compile-time constants: iow, they get turned into direct jumps.
Tadaa. You actually get good code generation, and you use *less*
dynamic memory since you don't have to have this pointer to the
descriptor.
Linus
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