Re: [PATCH] rcu: small rcu_dereference doc update

Pranith Kumar
2015-04-17T15:55:54Z

On Fri, Apr 17, 2015 at 6:33 AM, Milos Vyletel <milos@redhat.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@citrix.com>
> Signed-off-by: Milos Vyletel <milos@redhat.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.

--
Pranith