Re: [PATCH 1/1] rculist: Made list_first_entry_rcu usable

Michel Machado
2012-04-09T22:08:50Z

On Mon, 2012-04-09 at 14:24 -0700, Paul E. McKenney wrote:
> On Mon, Apr 02, 2012 at 09:42:34PM -0400, Michel Machado wrote:
> > The macro list_first_entry_rcu assumed that the passed list is not empty
> > as its counterpart list_first_entry does. However, one can test that a
> > list is not empty with list_empty before calling list_first_entry,
> > whereas neither exists list_empty_rcu, nor is advisable to add it as the
> > example below shows.
> >
> > Assuming that list_empty_rcu is available, one could write the following
> > snippet:
> >
> > if (!list_empty_rcu(mylist)) {
> > struct foo *bar = list_first_entry_rcu(mylist, struct foo,
> > list_member);
> > do_something(bar);
> > }
> >
> > The problem with this snippet is the following racing condition: the
> > list may not be empty when list_empty_rcu checks it, but it may be when
> > list_first_entry_rcu rereads the ->next pointer.
> >
> > This patch cannot break any upstream code because list_first_entry_rcu
> > is not being used anywhere in the kernel (tested with grep(1)), and
> > external code that uses it is probably broken already.
>
> Hello, Michel,
>
> Interesting point!
>
> Are you intending to use list_first_entry_rcu()? If not, perhaps the
> best thing to do is to remove it.
>
> Thanx, Paul

Hi Paul,

I'd rather keep list_first_entry_rcu(). I've already used it twice in
the project I'm working on
(https://github.com/AltraMayor/XIA-for-Linux), and I expect to submit
this work upstream once it reaches reasonable quality as you can check
in the roadmap available here:

https://github.com/AltraMayor/XIA-for-Linux/wiki/Roadmap#wiki-Making_into_Linus_source_tree

Not to mention that, given the subtlety of the problem, removing
list_first_entry_rcu() may introduce the same bug whenever someone tries
to mimic list_first_entry(), and having it in the kernel helps to guide
those with an example.

[ ]'s
Michel Machado

>
> > Signed-off-by: Michel Machado <michel@digirati.com.br>
> > CC: Dipankar Sarma <dipankar@in.ibm.com>
> > CC: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> > ---
> > Please CC my e-mail address while replying this message because I don't
> > subscribe this mailing list due to its high volume; thanks.
> >
> > diff --git a/include/linux/rculist.h b/include/linux/rculist.h
> > index d079290..866d3ec 100644
> > --- a/include/linux/rculist.h
> > +++ b/include/linux/rculist.h
> > @@ -233,13 +233,16 @@ static inline void list_splice_init_rcu(struct
> > list_head *list,
> > * @type: the type of the struct this is embedded in.
> > * @member: the name of the list_struct within the struct.
> > *
> > - * Note, that list is expected to be not empty.
> > + * Note that if the list is empty, it returns NULL.
> > *
> > * This primitive may safely run concurrently with the _rcu
> > list-mutation
> > * primitives such as list_add_rcu() as long as it's guarded by
> > rcu_read_lock().
> > */
> > #define list_first_entry_rcu(ptr, type, member) \
> > - list_entry_rcu((ptr)->next, type, member)
> > + ({struct list_head *__ptr = ptr; \
> > + struct list_head __rcu *__next = list_next_rcu(__ptr); \
> > + likely(__ptr != __next) ? container_of(__next, type, member) : NULL;
> > \
> > + })
> >
> > /**
> > * list_for_each_entry_rcu - iterate over rcu list of given type
> >
> >
>