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

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Date:	Fri, 17 Dec 2010 12:51:44 -0800
From:	David Daney <ddaney@...iumnetworks.com>
To:	Jason Baron <jbaron@...hat.com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	hpa@...or.com, rostedt@...dmis.org, mingo@...e.hu,
	tglx@...utronix.de, andi@...stfloor.org, roland@...hat.com,
	rth@...hat.com, masami.hiramatsu.pt@...achi.com,
	fweisbec@...il.com, avi@...hat.com, davem@...emloft.net,
	sam@...nborg.org, michael@...erman.id.au,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH/RFC 1/2] jump label: make enable/disable o(1)

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@...radead.org) 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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