Date:	Mon, 09 Apr 2012 18:42:33 -0400
From:	Michel Machado <michel@...irati.com.br>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Dipankar Sarma <dipankar@...ibm.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] rculist: Made list_first_entry_rcu usable

On Mon, 2012-04-09 at 15:22 -0700, Paul E. McKenney wrote:
> On Mon, Apr 09, 2012 at 06:08:42PM -0400, Michel Machado wrote:
> > 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.
> 
> Actually, list_first_entry_rcu() really does mimic list_first_entry()
> from what I can see.  Both of them require that the list be non-empty,
> which can be checked via !list_empty().
> 
> Or is list_first_entry() being converted to check for an empty list?
> 
> We really do need both list_first_entry() and list_first_entry_rcu()
> to have the same semantics on empty lists, I am sure you would agree.
> 
> 							Thanx, Paul

Yes, the current list_first_entry_rcu() does mimic list_first_entry(),
and that's the reason the problem is there. A list without RCU readers
would have list_empty() _and_ list_first_entry() protected by a lock, so
the reread of the ->next pointer isn't an issue there, but it's not the
case for a list with RCU readers.

I agree that having the same semantics for both is the perfect solution,
but list_first_entry() already has many users in the kernel. My patch is
a compromise to not have this bug lurking around, but I see that the
names may cause confusion. Would you be comfortable with removing the
current list_first_entry_rcu(), and adding my version renamed to
list_first_entry_if_not_empty_rcu() (or a shorter name)?

> 
> > [ ]'s
> > Michel Machado
> > 
> > > 
> > > > Signed-off-by: Michel Machado <michel@...irati.com.br>
> > > > CC: Dipankar Sarma <dipankar@...ibm.com>
> > > > CC: "Paul E. McKenney" <paulmck@...ux.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
> > > > 
> > > > 
> > > 
> > 
> 

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