Date: Mon, 3 Aug 2015 22:06:46 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        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, 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
Message-ID: <20150803200646.GF25159@twins.programming.kicks-ass.net>
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On Mon, Aug 03, 2015 at 04:05:33PM -0400, Steven Rostedt wrote:
> On Thu, 30 Jul 2015 17:40:01 +0200
> Peter Zijlstra <peterz@infradead.org> wrote:
> 
> > 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.
> 
> Note, mutex does do this for the optimistic spin. See
> mutex_try_to_aquire().

Right but that's mutex_lock(). mutex_trylock() does not.
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