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