Re: [PATCH] rcu: small rcu_dereference doc update

Paul E. McKenney
2015-04-17T14:06:48Z

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.

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
>