Date: Thu, 30 Jul 2015 17:40:01 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, jiangshanlai@gmail.com,
        dipankar@in.ibm.com, akpm@linux-foundation.org,
        mathieu.desnoyers@efficios.com, josh@joshtriplett.org,
        tglx@linutronix.de, rostedt@goodmis.org, dhowells@redhat.com,
        edumazet@google.com, dvhart@linux.intel.com, fweisbec@gmail.com,
        oleg@redhat.com, bobby.prani@gmail.com, dave@stgolabs.net,
        waiman.long@hp.com
Subject: Re: [PATCH tip/core/rcu 19/19] rcu: Add fastpath bypassing funnel
 locking
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On Thu, Jul 30, 2015 at 08:34:52AM -0700, Paul E. McKenney wrote:
> > If the extra read before the cmpxchg() does not hurt, we should do the
> > same for mutex and make the above redundant.
> 
> I am pretty sure that different hardware wants it done differently.  :-/
> So I agree that hard data would be good.
> 
> I could probably further optimize the RCU code by checking for a
> single-node tree, but I am not convinced that this is worthwhile.
> However, skipping three cache misses in the uncontended case is
> definitely worthwhile, hence this patch.  ;-)

I was mostly talking about the !mutex_is_locked() && mutex_try_lock()
thing. The fast path thing makes sense.
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