Re: [PATCH] rcu: small rcu_dereference doc update

Paul E. McKenney
2015-04-17T14:24:46Z

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@linux.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@citrix.com>
> > > Signed-off-by: Milos Vyletel <milos@redhat.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
> > >
>