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

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Date:	Thu, 16 Dec 2010 20:10:02 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jason Baron <jbaron@...hat.com>
Cc:	hpa@...or.com, rostedt@...dmis.org, mingo@...e.hu,
	mathieu.desnoyers@...ymtl.ca, 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/RFC 1/2] jump label: make enable/disable o(1)

On Thu, 2010-12-16 at 13:25 -0500, Jason Baron wrote:
> Previously, I allowed any variable type to be used as the 'key' for
> the jump label. However, by enforcing a type, we can make use of the
> contents of the 'key'. This patch thus introduces:
> 
> struct jump_label_key {
>        void *ptr;
> };
> 
> The 'ptr' is used a pointer into the jump label table of the
> corresponding addresses that need to be updated. Thus, when jump labels
> are enabled/disabled we have a constant time algorithm. There is no
> longer any hashing.
> 
> When jump lables are disabled we simply have:
> 
> struct jump_label_key {
>         int state;
> };
> 
> I've also defined an analogous structure for ref counted jump labels as
> per a request from Peter.
> 
> struct jump_label_keyref {
>        void *ptr;
> };
> 
> And for the jump labels disabled case:
> 
> 
> struct jump_label_keyref {
>         atomic_t refcount;
> };
> 
> The reason I've introduced an additional structure for the reference counted
> jump labels is twofold:
> 
> 1) For the jump labels disabled case, reference counted jump labels use an
> atomic_read(). I didn't want to impact the jump labels disabled case for
> tracepoints which simply accesses an 'int'.
> 
> 2) By introducing a second type, we have two parallel APIs:
> 
> extern void jump_label_enable(struct jump_label_key *key);
> extern void jump_label_disable(struct jump_label_key *key);
> 
> static inline void jump_label_inc(struct jump_label_keyref *key)
> static inline void jump_label_dec(struct jump_label_keyref *key)
> 
> In this way, we can't mix up the reference counted API, with the straight
> enable/disable API since they accept different types.

But why do we want to have two APIs?
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