linux-kernel - Re: [RFC][PATCH 2/5] signal: Add rwlock to protect sighand->action

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Date:	Fri, 30 Sep 2011 17:25:17 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Matt Fleming <matt@...sole-pimps.org>
Cc:	Oleg Nesterov <oleg@...hat.com>, Tejun Heo <tj@...nel.org>,
	linux-kernel@...r.kernel.org, Tony Luck <tony.luck@...el.com>,
	Matt Fleming <matt.fleming@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Anirudh Badam <abadam@...princeton.edu>
Subject: Re: [RFC][PATCH 2/5] signal: Add rwlock to protect sighand->action

On Fri, 2011-09-30 at 16:12 +0100, Matt Fleming wrote:
> 
> As sighand->action is read much more frequently than written a rwlock
> makes the most sense here. 

Ha! you would think so, but then you'd forget that
read_lock()+read_unlock() are atomic ops modifying the lock state as
well. Furthermore rwlocks aren't fair by any means.

Therefore rwlock_t should never be used, use a spinlock_t possibly in
combination with RCU or seqcount etc..
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