linux-kernel - Re: [PATCH tip/core/rcu 08/10] rcu: Add a TINY_PREEMPT

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Date:	Tue, 17 Aug 2010 12:33:58 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
	mingo@...e.hu, laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, josh@...htriplett.org,
	dvhltc@...ibm.com, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH tip/core/rcu 08/10] rcu: Add a TINY_PREEMPT_RCU

On Tue, Aug 17, 2010 at 12:25:25PM -0400, Mathieu Desnoyers wrote:
> * Steven Rostedt (rostedt@...dmis.org) wrote:
> > On Tue, 2010-08-17 at 12:04 -0400, Steven Rostedt wrote:
> > 
> > > > Then we could go for the simpler:
> > > > 
> > > >   	--t->rcu_read_lock_nesting;
> > > >  	barrier();
> > > >         if (t->rcu_read_lock_nesting == 0 &&
> > > >              unlikely((t->rcu_read_unlock_special))
> > > 
> > > Yeah, that's what I meant, I was too lazy to remove the ACCESS_ONCE()
> > > from the cut and paste I did.
> > > 
> > > > 
> > > > Which puts a constraint across all memory accesses. I'd be fine with
> > > > that if you are afraid of too much micro-optimization (e.g. my
> > > > barrier2(a, b) proposal).
> > > 
> > > Not afraid, but just too much code for a simple solution.
> > 
> > IOW,
> > 
> > I think just pulling out the '--' and adding the barrier() is the proper
> > solution here. Compiler barriers are rather cheap.
> > 
> > Can we all agree on this solution?
> 
> Given that we already have a barrier() at the beginning of
> rcu_read_unlock(), adding a second one will not have much more global
> optimisation impact than what is already there. I'm personally fine with
> this solution. Let's see what others have to say about this.

Thank you both for the optimization work -- the read-side primitives do
need to be fast.  And the barrier() approach generates decent code, on
some systems better than the original.  So the second barrier wins.  ;-)

							Thanx, Paul
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