Messages in this thread | | | Date | Mon, 9 Apr 2012 16:11:04 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH 1/1] rculist: Made list_first_entry_rcu usable |
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On Mon, Apr 09, 2012 at 06:42:33PM -0400, Michel Machado wrote: > 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)?
Dropping list_first_entry_rcu() in favor of something that reliably checks for NULL makes sense, but yes, a shorter name would be good. ;-)
The comment headers should document the problem as well.
Thanx, Paul
> > > [ ]'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 > > > > > > > > > > > > > > > > > > > >
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