Re: [PATCH/RFC 1/2] jump label: make enable/disable o(1)

[David Daney]

On 12/17/2010 12:07 PM, Jason Baron wrote:
> On Thu, Dec 16, 2010 at 09:56:25PM +0100, Peter Zijlstra wrote:
> > On Thu, 2010-12-16 at 15:50 -0500, Mathieu Desnoyers wrote:
> > > * Peter Zijlstra (peterz@xxxxxxxxxxxxx) wrote:
> > > > On Thu, 2010-12-16 at 15:36 -0500, Mathieu Desnoyers wrote:
> > > > > Tracepoints keep their own reference counts for enable/disable, so a
> > > > > simple "enable/disable" is fine as far as tracepoints are concerned. Why
> > > > > does perf need that refcounting done by the static jumps ?
> > > >
> > > > Because the refcount is all we have... Why not replace that tracepoint
> > > > refcount with the jumplabel thing?
> > >
> > > The reason why tracepoints need to keep their own refcount is because
> > > they support dynamically loadable modules, and hence the refcount must
> > > be kept outside of the modules, in a table internal to tracepoints,
> > > so we can attach a probe to a yet unloaded module. Therefore, relying on
> > > this lower level jump label to keep the refcount is not appropriate for
> > > tracepoints, because the refcount only exists when the module is live.
> >
> > That's not a logical conclusion, you can keep these jump_label keys
> > outside of the module just fine.
> >
> > > I know that your point of view is "let users of modules suffer", but
> > > this represents a very large portion of Linux users I am not willing to
> > > let suffer knowingly.
> >
> > Feh, I'd argue to remove this special tracepoint crap, the only
> > in-kernel user (ftrace) doesn't even make use of it. This weird ass
> > tracepoint semantic being different from the ftrace trace_event
> > semantics has caused trouble before.
>
> Hi,
>
> since atomic_t is just an 'int' from include/linux/types.h, so for all
> arches. We can cast any refernces to an atomic_t in
> include/linux/jump_label_ref.h

Not acceptable I would think.

How about:

union fubar {
   int key_as_non_atomic;
   atomic_t key_as_atomic;
};

Now explain the exact semantics of this thing including how you 
guarantee no conflicting accesses *ever* occur.


> So for when jump labels are disabled case we could have
> one struct:
>
> struct jump_label_key {
> 	int state;
> }
>
> and then we could then have (rough c code):
>
> jump_label_enable(struct jump_label_key *key)
> {
> 	key->state = 1;
> }
>
> jump_label_disable(struct jump_label_key *key)
> {
> 	key->state = 0;
> }
>
> jump_label_inc(struct jump_label_key *key)
> {
> 	atomic_inc((atomic_t *)key)
> }
>
> jump_label_dec(struct jump_label_key *key)
> {
> 	atomic_dec((atomic_t *)key)
> }
>
> bool unlikely_switch(struct jump_label_key *key)
> {
> 	if (key->state)
> 		return true;
> 	return false;
> }
>
> bool unlikely_switch_atomic(struct jump_label_key *key)
> {
> 	if (atomic_read((atomic_t *)key)
> 		return true;
> 	return false;
> }
>
> 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).

David Daney

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