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

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Message-ID: <20110211221550.GA9148@redhat.com>
Date:	Fri, 11 Feb 2011 17:15:50 -0500
From:	Jason Baron <jbaron@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	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,
	ddaney@...iumnetworks.com, michael@...erman.id.au,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2] jump label: 2.6.38 updates

On Fri, Feb 11, 2011 at 10:38:17PM +0100, Peter Zijlstra 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).
> 

a bit of history...

For the disabled jump label case, we didn't want to incur an atomic_read() to
check if the branch was enabled.

So, I separated the API, to have one for the non-atomic case, and one
for the atomic case. Nobody liked that.

So now, I'm proposing to leave the core API based around a non-atomic
variable, and have any callers that want to use this atomic interface,
to call into the non-atomic interface. If another user besides perf
wants to use the same type of atomic interface, we can re-visit the
decsion? 

thanks,

-Jason
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