Re: [PATCH/RFC 1/2] jump label: make enable/disable o(1)
From: Steven Rostedt
Date: Fri Dec 17 2010 - 16:12:35 EST
On Fri, 2010-12-17 at 12:51 -0800, David Daney wrote:
> On 12/17/2010 12:07 PM, Jason Baron wrote:
> Not acceptable I would think.
>
> How about:
>
> union fubar {
> int key_as_non_atomic;
> atomic_t key_as_atomic;
> };
I don't even like this union.
>
> Now explain the exact semantics of this thing including how you
> guarantee no conflicting accesses *ever* occur.
I don't like the mixed semantics at all.
>
>
> > So for when jump labels are disabled case we could have
> > one struct:
> >
> > struct jump_label_key {
atomic_t state;
> > }
> >
> > and then we could then have (rough c code):
> >
> > jump_label_enable(struct jump_label_key *key)
> > {
if (atomic_read(&key->state))
return;
atomic_inc(&key->state);
> > }
> >
> > jump_label_disable(struct jump_label_key *key)
> > {
if (!atomic_read(&key->state))
return;
atomic_dec(&key->state);
WARN_ON(atomic_read(&key->state);
> > }
> >
> > jump_label_inc(struct jump_label_key *key)
> > {
atomic_inc(&key->state)
> > }
> >
> > jump_label_dec(struct jump_label_key *key)
> > {
atomic_dec((&key->state)
> > }
> >
> > bool unlikely_switch(struct jump_label_key *key)
> > {
if (atomic_read(&key->state))
> > return true;
> > return false;
> > }
> >
There, now you are guaranteed that you have proper semantics.
> >
> > can we agree on something like this?
>
> I get a sick feeling whenever casting is used to give types with well
> defined semantics (atomic_t) poorly defined semantics (your usage).
Exactly, I like to avoid (void*) anything or even worse, casting one
type to another for some strange semantics. This is guaranteed nightmare
of maintenance.
-- Steve
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