linux-kernel - Re: [PATCH 0/2] jump label: 2.6.38 updates

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Message-ID: <BLU0-SMTP4078FDA13D3CAAEA18602E96EF0@phx.gbl>
Date:	Fri, 11 Feb 2011 17:20:25 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Jason Baron <jbaron@...hat.com>, 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,
	ddaney@...iumnetworks.com, michael@...erman.id.au,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2] jump label: 2.6.38 updates

* Peter Zijlstra (peterz@...radead.org) wrote:
> On Fri, 2011-02-11 at 16:13 -0500, Mathieu Desnoyers wrote:
> > 
> > Thoughts ? 
> 
>  #if defined(CC_HAVE_ASM_GOTO) && defined(CONFIG_JUMP_LABEL)
> +
> +struct jump_label_key {
> +       void *ptr;
> +};
> 
>  struct jump_label_entry {
>         struct hlist_node hlist;
>         struct jump_entry *table;
> -       int nr_entries;
>         /* hang modules off here */
>         struct hlist_head modules;
>         unsigned long key;
> +       u32 nr_entries;
> +       int refcount;
>  };
> 
> #else
> 
> +struct jump_label_key {
> +       int state;
> +};
> 
> #endif
> 
> So why can't we make that jump_label_entry::refcount and
> jump_label_key::state an atomic_t and be done with it?
> 
> Then the enabled case uses if (atomic_inc_return(&key->ptr->refcount) ==
> 1), and the disabled atomic_inc(&key->state).
> 

OK, by "enabled" you mean #if defined(CC_HAVE_ASM_GOTO) &&
defined(CONFIG_JUMP_LABEL), and "disabled", the #else.

I guess the only downside is the extra volatile for the atomic_read for
the fallback case, which is not really much of problem realistically
speaking: anyway, the volatile is a good thing to have in the fallback
case to force the compiler to re-read the variable. Let's go with your
idea.

Thanks,

Mathieu


-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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