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Commit 2a855b64 authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

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: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
parent 829511d8

include/linux/rculist.h

+21 −2
Original line number Diff line number Diff line
@@ -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.