linux-kernel - Re: [PATCH tip/core/rcu 2/3] documentation: Update rcu_dereference.txt based on WG21 discussions

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Date:	Wed, 13 May 2015 18:33:10 +0000 (UTC)
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, josh@...htriplett.org,
	tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org,
	dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com,
	fweisbec@...il.com, oleg@...hat.com,
	bobby prani <bobby.prani@...il.com>
Subject: Re: [PATCH tip/core/rcu 2/3] documentation: Update
 rcu_dereference.txt based on WG21 discussions

----- Original Message -----
> From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> 
> This commit provides another caveat for the care and feeding of pointers
> returned by rcu_dereference() that was pointed out in discussions within
> the C++ standards committee.
> 
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>

BTW, reading through rcu_dereference.txt, I found a nit:

operatiors -> operators

Very interesting reading :)

Thanks!

Mathieu

> ---
>  Documentation/RCU/rcu_dereference.txt | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/Documentation/RCU/rcu_dereference.txt
> b/Documentation/RCU/rcu_dereference.txt
> index ceb05da5a5ac..2d05c9241a33 100644
> --- a/Documentation/RCU/rcu_dereference.txt
> +++ b/Documentation/RCU/rcu_dereference.txt
> @@ -193,6 +193,11 @@ o	Be very careful about comparing pointers obtained from
>  		pointer.  Note that the volatile cast in rcu_dereference()
>  		will normally prevent the compiler from knowing too much.
>  
> +		However, please note that if the compiler knows that the
> +		pointer takes on only one of two values, a not-equal
> +		comparison will provide exactly the information that the
> +		compiler needs to deduce the value of the pointer.
> +
>  o	Disable any value-speculation optimizations that your compiler
>  	might provide, especially if you are making use of feedback-based
>  	optimizations that take data collected from prior runs.  Such
> --
> 1.8.1.5
> 
> 

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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