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rcu: Make list_splice_init_rcu() account for RCU readers 

The list_splice_init_rcu() function allows a list visible to RCU readers
to be spliced into another list visible to RCU readers.  This is OK,
except for the use of INIT_LIST_HEAD(), which does pointer updates
without doing anything to make those updates safe for concurrent readers.

Of course, most of the time INIT_LIST_HEAD() is being used in reader-free
contexts, such as initialization or cleanup, so it is OK for it to update
pointers in an unsafe-for-RCU-readers manner.  This commit therefore
creates an INIT_LIST_HEAD_RCU() that uses ACCESS_ONCE() to make the updates
reader-safe.  The reason that we can use ACCESS_ONCE() instead of the more
typical rcu_assign_pointer() is that list_splice_init_rcu() is updating the
pointers to reference something that is already visible to readers, so
that there is no problem with pre-initialized values.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit is contained in:
Paul E. McKenney 2013-08-23 09:40:42 -07:00
parent 829511d8aa
commit 2a855b644c
1 changed files with 21 additions and 2 deletions
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23
include/linux/rculist.h
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@ -18,6 +18,21 @@
* be used anywhere you would want to use a list_empty_rcu().
*/
/*
* INIT_LIST_HEAD_RCU - Initialize a list_head visible to RCU readers
* @list: list to be initialized
*
* You should instead use INIT_LIST_HEAD() for normal initialization and
* cleanup tasks, when readers have no access to the list being initialized.
* However, if the list being initialized is visible to readers, you
* need to keep the compiler from being too mischievous.
*/
static inline void INIT_LIST_HEAD_RCU(struct list_head *list)
{
ACCESS_ONCE(list->next) = list;
ACCESS_ONCE(list->prev) = list;
}
/*
* return the ->next pointer of a list_head in an rcu safe
* way, we must not access it directly
@ -191,9 +206,13 @@ static inline void list_splice_init_rcu(struct list_head *list,
if (list_empty(list))
return;
/* "first" and "last" tracking list, so initialize it. */
/*
* "first" and "last" tracking list, so initialize it. RCU readers
* have access to this list, so we must use INIT_LIST_HEAD_RCU()
* instead of INIT_LIST_HEAD().
*/
INIT_LIST_HEAD(list);
INIT_LIST_HEAD_RCU(list);
/*
* At this point, the list body still points to the source list.