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

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Message-ID: <20150417142436.GN23685@linux.vnet.ibm.com>
Date:	Fri, 17 Apr 2015 07:24:36 -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] rcu: small rcu_dereference doc update

On Fri, Apr 17, 2015 at 10:13:50AM -0400, Steven Rostedt wrote:
> On Fri, 17 Apr 2015 07:06:38 -0700
> "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:
> 
> > 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.
> 
> Before you queue it, there's a few articles that are screaming to be
> present...

OK, I removed it.  Milos, when you send me a version that Steve is happy
with, I will requeue it.

							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.
> 
> 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.
> 
> -- Steve
> 
> > > 
> > >  	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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