[PATCH v2] rcu: small rcu_dereference doc update

[PATCH v2] rcu: small rcu_dereference doc update

[Milos Vyletel]

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>
---
 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..16622c9 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 the same pointer will be returned
+	if an update happened while in the 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

Re: [PATCH v2] rcu: small rcu_dereference doc update

[Steven Rostedt]

On Fri, 17 Apr 2015 16:38:04 +0200
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.
> 
> Reported-by: Jeff Haran <jeff.haran@citrix.com>
> Signed-off-by: Milos Vyletel <milos@redhat.com>

Reviewed-by: Steven Rostedt <rostedt@goodmis.org>

-- Steve

> ---
>  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..16622c9 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 the same pointer will be returned
> +	if an update happened while in the 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.

Re: [PATCH v2] rcu: small rcu_dereference doc update

[Paul E. McKenney]

On Fri, Apr 17, 2015 at 10:41:30AM -0400, Steven Rostedt wrote:
> On Fri, 17 Apr 2015 16:38:04 +0200
> 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.
> > 
> > Reported-by: Jeff Haran <jeff.haran@citrix.com>
> > Signed-off-by: Milos Vyletel <milos@redhat.com>
> 
> Reviewed-by: Steven Rostedt <rostedt@goodmis.org>

Queued for 4.2, thank you both!

							Thanx, Paul

> -- Steve
> 
> > ---
> >  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..16622c9 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 the same pointer will be returned
> > +	if an update happened while in the 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.
>