linux-kernel - Re: [PATCH 0/3] static keys: fix test/set races

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Message-ID: <20130702080306.GD21726@dyad.programming.kicks-ass.net>
Date:	Tue, 2 Jul 2013 10:03:06 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jason Baron <jbaron@...mai.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	"rostedt@...dmis.org" <rostedt@...dmis.org>,
	"andi@...stfloor.org" <andi@...stfloor.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/3] static keys: fix test/set races

On Mon, Jul 01, 2013 at 12:12:11AM -0400, Jason Baron wrote:
> 
> Yes, I agree that 'higher' level locking may be required for some callers of
> the newly proposed interface. However, I do think that the
> static_key_slow_set_true()/false() provides a nice abstraction for some
> callers, while addressing test/set() races, by making that sequence atomic.
> 
> I view the proposed inteface of set_true()/set_false() as somewhat analogous
> to an atomic_set() call. In the same way, the current
> static_key_slow_inc()/dec() are analogous to atomic_inc()/dec().
> 
> It arguably makes the code code a bit more readable, transforming sequences
> such as:
> 
> if (!static_key_enabled(&control_var))
>         static_key_slow_inc(&control_var);
> 
> into:
> 
>         static_key_slow_set_true(&control_var);
> 
> 
> I see at least 3 users of static_keys in the tree which I think would
> benefit  from this transformation. The 2 attached with this series, and the
> usage in kernel/tracepoint.c.

I tend to agree with Jason here. I also dont' think the scheduler needs this;
but the new API is more usable for binary switches as opposed to the refcount
thing.


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