On Wed, Aug 23, 2023 at 09:24:21AM -0400, Joel Fernandes wrote:
> On Tue, Aug 22, 2023 at 7:54 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> >
> > On Tue, Aug 22, 2023 at 07:28:34PM -0400, Joel Fernandes wrote:
> > > On Tue, Aug 22, 2023 at 4:04 PM Matthew Wilcox (Oracle)
> > > <willy@xxxxxxxxxxxxx> wrote:
> > > >
> > > > More explicitly state what is, and what is not guaranteed to those
> > > > who iterate a list while protected by RCU.
> > > >
> > > > Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> > > > ---
> > > > Documentation/RCU/listRCU.rst | 12 ++++++++++++
> > > > 1 file changed, 12 insertions(+)
> > > >
> > > > diff --git a/Documentation/RCU/listRCU.rst b/Documentation/RCU/listRCU.rst
> > > > index bdc4bcc5289f..aff1394f6d93 100644
> > > > --- a/Documentation/RCU/listRCU.rst
> > > > +++ b/Documentation/RCU/listRCU.rst
> > > > @@ -8,6 +8,18 @@ One of the most common uses of RCU is protecting read-mostly linked lists
> > > > that all of the required memory ordering is provided by the list macros.
> > > > This document describes several list-based RCU use cases.
> > > >
> > > > +When iterating a list while holding the rcu_read_lock(), writers may
> > > > +modify the list. The reader is guaranteed to see all of the elements
> > > > +which were added to the list before they acquired the rcu_read_lock()
> > > > +and are still on the list when they drop the rcu_read_unlock().
> > > > +Elements which are added to, or removed from the list may or may not
> > > > +be seen. If the writer calls list_replace_rcu(), the reader may see
> > > > +either the old element or the new element; they will not see both,
> > > > +nor will they see neither.
> > >
> > > Until here,
> > > Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> > >
> > > > +There is no equivalent of list_for_each_entry_reverse(); RCU lists
> > > > +may only be walked forwards.
> > > > +
> > >
> > > Is there a need to mention that? If it changes in the future then the
> > > docs go stale.
> >
> > I don't know if there's a need to mention it. It was something I noticed
> > while reviewing the code that I didn't see documented anywhere.
> >
> > I don't really care; happy to send a v2 without it, or you can just
> > apply the first paragraph.
>
> That sounds good to me, Paul do you want to apply it with my Review
> tag and without the last para?
Here you two go, and thank you both!
Thanx, Paul
------------------------------------------------------------------------
commit 06f9695d1b62163036581722f29fef2d93aa7909
Author: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
Date: Tue Aug 22 21:04:02 2023 +0100
rcu: Describe listRCU read-side guarantees
More explicitly state what is, and what is not guaranteed to those
who iterate a list while protected by RCU.
[ paulmck: Apply Joel Fernandes feedback. ]
Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
diff --git a/Documentation/RCU/listRCU.rst b/Documentation/RCU/listRCU.rst
index bdc4bcc5289f..ed5c9d8c9afe 100644
--- a/Documentation/RCU/listRCU.rst
+++ b/Documentation/RCU/listRCU.rst
@@ -8,6 +8,15 @@ One of the most common uses of RCU is protecting read-mostly linked lists
that all of the required memory ordering is provided by the list macros.
This document describes several list-based RCU use cases.
+When iterating a list while holding the rcu_read_lock(), writers may
+modify the list. The reader is guaranteed to see all of the elements
+which were added to the list before they acquired the rcu_read_lock()
+and are still on the list when they drop the rcu_read_unlock().
+Elements which are added to, or removed from the list may or may not
+be seen. If the writer calls list_replace_rcu(), the reader may see
+either the old element or the new element; they will not see both,
+nor will they see neither.
+
Example 1: Read-mostly list: Deferred Destruction
-------------------------------------------------
