On Thu, Dec 12, 2024 at 04:42:54PM -0800, Paul E. McKenney wrote: > On Fri, Dec 13, 2024 at 12:03:43AM +0100, Christian Brauner wrote: > > Currently there is no primitive for retrieving the previous list member. > > To do this we need a new deletion primitive that doesn't poison the prev > > pointer and a corresponding retrieval helper. Note that it is not valid > > to ues both list_del_rcu() and list_bidir_del_rcu() on the same list. > > > > Suggested-by: "Paul E. McKenney" <paulmck@xxxxxxxxxx> > > Signed-off-by: Christian Brauner <brauner@xxxxxxxxxx> > > One additional nit below. With that fixed: > > Reviewed-by: Paul E. McKenney <paulmck@xxxxxxxxxx> Thansk, Paul! > > > --- > > include/linux/rculist.h | 47 +++++++++++++++++++++++++++++++++++++++++++++++ > > 1 file changed, 47 insertions(+) > > > > diff --git a/include/linux/rculist.h b/include/linux/rculist.h > > index 14dfa6008467e803d57f98cfa0275569f1c6a181..270a9ee2f7976b1736545667973265a3bfb7ec41 100644 > > --- a/include/linux/rculist.h > > +++ b/include/linux/rculist.h > > @@ -30,6 +30,17 @@ static inline void INIT_LIST_HEAD_RCU(struct list_head *list) > > * way, we must not access it directly > > */ > > #define list_next_rcu(list) (*((struct list_head __rcu **)(&(list)->next))) > > +/* > > + * Return the ->prev pointer of a list_head in an rcu safe way. Don't > > + * access it directly. > > + * > > + * Any list traversed with list_bidir_prev_rcu() must never use > > + * list_del_rcu(). Doing so will poison the ->prev pointer that > > + * list_bidir_prev_rcu() relies on, which will result in segfaults. > > + * To prevent these segfaults, use list_bidir_del_rcu() instead > > + * of list_del_rcu(). > > + */ > > +#define list_bidir_prev_rcu(list) (*((struct list_head __rcu **)(&(list)->prev))) > > > > /** > > * list_tail_rcu - returns the prev pointer of the head of the list > > @@ -158,6 +169,42 @@ static inline void list_del_rcu(struct list_head *entry) > > entry->prev = LIST_POISON2; > > } > > > > +/** > > + * list_bidir_del_rcu - deletes entry from list without re-initialization > > + * @entry: the element to delete from the list. > > + * > > + * In contrast to list_del_rcu() doesn't poison the prev pointer thus > > + * allowing backwards traversal via list_bidir_prev_rcu(). > > + * > > + * Note: list_empty() on entry does not return true after this because > > + * the entry is in a special undefined state that permits RCU-based > > + * lockfree reverse traversal. In particular this means that we can not > > + * poison the forward and backwards pointers that may still be used for > > + * walking the list. > > + * > > + * The caller must take whatever precautions are necessary (such as > > + * holding appropriate locks) to avoid racing with another list-mutation > > + * primitive, such as list_bidir_del_rcu() or list_add_rcu(), running on > > + * this same list. However, it is perfectly legal to run concurrently > > + * with the _rcu list-traversal primitives, such as > > + * list_for_each_entry_rcu(). > > + * > > + * Noe that the it is not allowed to use list_del_rcu() and > > + * list_bidir_del_rcu() on the same list. > > I am guessing that the above paragraph is a leftover? Indeed, fixed!
