rcu: Make list_splice_init_rcu() account for RCU readers (2a855b64) · Commits · 戴 / test

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>

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.

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