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

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Date:	Fri, 17 Apr 2015 09:15:27 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Pranith Kumar <bobby.prani@...il.com>
Cc:	Milos Vyletel <milos@...hat.com>,
	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 11:55:20AM -0400, Pranith Kumar wrote:
> On Fri, Apr 17, 2015 at 6:33 AM, 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.
> 
> Might as well make it more explicit with an example then. See below:
> 
> >
> > Reported-by: Jeff Haran <jeff.haran@...rix.com>
> > Signed-off-by: Milos Vyletel <milos@...hat.com>
> > ---
> >  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.
> >
> 
> An example like follows:
> 
> struct some_ds {
>         int data;
>         bool ready;
> };
> 
> struct some_ds *p = ...;
> 
> rcu_read_lock();
> if (rcu_dereference(p->ready))
>     data = rcu_dereference(p->data); // bug
> rcu_read_unlock();
> 
> or some such.

Hello, Pranith!

Sounds like a good thought for a separate patch.  Please take a look
through the rest of the documentation -- this might well be the right
place for such an example, but there might well be a better place.
Is this issue mentioned in the checklist?  If not, another item might
be good.

							Thanx, Paul

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