Subject: Re: [PATCH/RFC 1/2] jump label: make enable/disable o(1)
From: Steven Rostedt <rostedt@goodmis.org>
To: David Daney <ddaney@caviumnetworks.com>
Cc: Jason Baron <jbaron@redhat.com>, Peter Zijlstra <peterz@infradead.org>,
        Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>, hpa@zytor.com,
        mingo@elte.hu, tglx@linutronix.de, andi@firstfloor.org,
        roland@redhat.com, rth@redhat.com, masami.hiramatsu.pt@hitachi.com,
        fweisbec@gmail.com, avi@redhat.com, davem@davemloft.net,
        sam@ravnborg.org, michael@ellerman.id.au, linux-kernel@vger.kernel.org
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Date: Fri, 17 Dec 2010 16:12:21 -0500
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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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