linux-kernel - Re: [PATCH v2] rcu: small rcu

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Date:	Fri, 17 Apr 2015 08:34:26 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Milos Vyletel <milos@...hat.com>,
	Josh Triplett <josh@...htriplett.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 v2] rcu: small rcu_dereference doc update

On Fri, Apr 17, 2015 at 10:41:30AM -0400, Steven Rostedt wrote:
> On Fri, 17 Apr 2015 16:38:04 +0200
> Milos Vyletel <milos@...hat.com> 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>
> 
> Reviewed-by: Steven Rostedt <rostedt@...dmis.org>

Queued for 4.2, thank you both!

							Thanx, Paul

> -- Steve
> 
> > ---
> >  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..16622c9 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 the same pointer will be returned
> > +	if an update happened while in the 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.
> 

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