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

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Date:	Fri, 17 Apr 2015 11:55:20 -0400
From:	Pranith Kumar <bobby.prani@...il.com>
To:	Milos Vyletel <milos@...hat.com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.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 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.

-- 
Pranith
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