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    SubjectRe: [PATCH 1/1] rculist: Replaced list_first_entry_rcu() with list_first_or_null_rcu()
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    On Wed, 2012-04-11 at 17:30 -0700, Paul E. McKenney wrote:
    > On Tue, Apr 10, 2012 at 02:07:40PM -0400, Michel Machado wrote:
    > > Replaced list_first_entry_rcu() with list_first_or_null_rcu() because
    > > list_first_entry_rcu() is not safe as one can find in the comment that
    > > this patch also adds.
    > >
    > > This patch incorporated Paul's suggestions to the previous version of
    > > this patch available here:
    > >
    > > https://lkml.org/lkml/2012/4/2/536
    > >
    > > 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.
    >
    > Thank you, Michel, I have queued this.
    >
    > However, in the future, could you please configure your email client to
    > avoid breaking lines and could you please also run scripts/checkpatch.pl
    > on future patches? Applying your patch took some hand-editing to merge
    > the broken lines and to remove trailing spaces on lines.
    >
    > Thanx, Paul
    >

    Sorry for the extra work, I'll pay more attention next time.

    [ ]'s
    Michel Machado

    > > Signed-off-by: Michel Machado <michel@digirati.com.br>
    > > CC: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
    > > CC: Dipankar Sarma <dipankar@in.ibm.com>
    > > ---
    > >
    > > diff --git a/include/linux/rculist.h b/include/linux/rculist.h
    > > index d079290..ef5da30 100644
    > > --- a/include/linux/rculist.h
    > > +++ b/include/linux/rculist.h
    > > @@ -228,18 +228,43 @@ static inline void list_splice_init_rcu(struct
    > > list_head *list,
    > > })
    > >
    > > /**
    > > - * list_first_entry_rcu - get the first element from a list
    > > + * Where are list_empty_rcu() and list_first_entry_rcu()?
    > > + *
    > > + * Implementing those functions following their counterparts
    > > list_empty() and
    > > + * list_first_entry() is not advisable because they lead to subtle race
    > > + * conditions as the following snippet shows:
    > > + *
    > > + * if (!list_empty_rcu(mylist)) {
    > > + * struct foo *bar = list_first_entry_rcu(mylist, struct foo,
    > > list_member);
    > > + * do_something(bar);
    > > + * }
    > > + *
    > > + * 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.
    > > + *
    > > + * Rereading the ->next pointer is not a problem for list_empty() and
    > > + * list_first_entry() because they would be protected by a lock that
    > > blocks
    > > + * writers.
    > > + *
    > > + * See list_first_or_null_rcu for an alternative.
    > > + */
    > > +
    > > +/**
    > > + * list_first_or_null_rcu - get the first element from a list
    > > * @ptr: the list head to take the element from.
    > > * @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)
    > > +#define list_first_or_null_rcu(ptr, 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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    > >
    >



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