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

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Date:	Tue, 17 Aug 2010 12:04:50 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	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, 2010-08-17 at 11:55 -0400, Mathieu Desnoyers wrote:

> > Heck, this is too much micro optimization. We could just be safe and do
> > the:
> >  	--t->rcu_read_lock_nesting;
> > 	barrier();
> >          if (ACCESS_ONCE(t->rcu_read_lock_nesting) == 0 &&
> >               unlikely((ACCESS_ONCE(t->rcu_read_unlock_special)))
> >  
> > And be done with it.
> 
> 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.

-- Steve


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