linux-kernel - Re: [PATCH] rcu: small rcu

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Message-ID: <20150417140637.GL23685@linux.vnet.ibm.com>
Date:	Fri, 17 Apr 2015 07:06:38 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Milos Vyletel <milos@...hat.com>
Cc:	Josh Triplett <josh@...htriplett.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Jonathan Corbet <corbet@....net>,
	"open list:READ-COPY UPDATE..." <linux-kernel@...r.kernel.org>,
	"open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
	jeff.haran@...rix.com
Subject: Re: [PATCH] rcu: small rcu_dereference doc update

On Fri, Apr 17, 2015 at 12:33:36PM +0200, Milos Vyletel wrote:
> Make a note stating that repeated calls of rcu_dereference() may not
> return the same pointer if update happens while in critical section.
> 
> Reported-by: Jeff Haran <jeff.haran@...rix.com>
> Signed-off-by: Milos Vyletel <milos@...hat.com>

Hmmm...  Seems like that should be obvious, but on the other hand,
I have been using RCU for more than twenty years, so my obviousness
sensors might need recalibration.

Queued for 4.2.

							Thanx, Paul

> ---
>  Documentation/RCU/whatisRCU.txt | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.txt
> index 88dfce1..82b1b2c 100644
> --- a/Documentation/RCU/whatisRCU.txt
> +++ b/Documentation/RCU/whatisRCU.txt
> @@ -256,7 +256,9 @@ rcu_dereference()
>  	If you are going to be fetching multiple fields from the
>  	RCU-protected structure, using the local variable is of
>  	course preferred.  Repeated rcu_dereference() calls look
> -	ugly and incur unnecessary overhead on Alpha CPUs.
> +	ugly, do not guarantee that same pointer will be returned
> +	if update happened while in critical section and incur
> +	unnecessary overhead on Alpha CPUs.
> 
>  	Note that the value returned by rcu_dereference() is valid
>  	only within the enclosing RCU read-side critical section.
> -- 
> 2.1.0
> 

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