linux-kernel - Re: [PATCH tip/core/rcu 19/19] rcu: Add fastpath bypassing funnel locking

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Message-ID: <20150730163427.GO25159@twins.programming.kicks-ass.net>
Date:	Thu, 30 Jul 2015 18:34:27 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	jiangshanlai@...il.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
	dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com,
	fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com,
	dave@...olabs.net, waiman.long@...com
Subject: Re: [PATCH tip/core/rcu 19/19] rcu: Add fastpath bypassing funnel
 locking

On Thu, Jul 30, 2015 at 08:34:52AM -0700, Paul E. McKenney wrote:
> On Thu, Jul 30, 2015 at 04:44:55PM +0200, Peter Zijlstra 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.  :-/

I think that most archs won't notice since any RmW includes a load of
that variable anyhow. The only case where it can matter is if the RmW is
done outside of the normal cache hierarchy -- like on Power, where the
ll/sc bypasses the L1.
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