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    SubjectRe: [PATCH 1/1] rculist: Made list_first_entry_rcu usable
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    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
    > >
    > >
    >



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