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SubjectRe: [PATCH/RFC 1/2] jump label: make enable/disable o(1)
On Fri, Dec 17, 2010 at 04:12:21PM -0500, Steven Rostedt 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;
> > > }
> > >
>

hmmm...we were trying to avoid having the atomic_read() for tracepoints
b/c of potential extra cost that Mathieu was concerned about.

> There, now you are guaranteed that you have proper semantics.
>
> > >

The other issue here was that jump_label.h gets included by
asm/atomic.h, so there a dependency issue to be addressed here as
well....

thanks,

-Jason



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