Messages in this thread | | | Date | Mon, 9 Apr 2012 14:24:40 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH 1/1] rculist: Made list_first_entry_rcu usable |
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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
> 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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