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Date: Tue, 24 Sep 2013 18:29:15 -0700 From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> To: linux-kernel@...r.kernel.org Cc: mingo@...nel.org, laijs@...fujitsu.com, dipankar@...ibm.com, akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com, josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com, darren@...art.com, fweisbec@...il.com, sbw@....edu, "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> Subject: [PATCH tip/core/rcu 05/11] rcu: Make list_splice_init_rcu() account for RCU readers From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> 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@...ux.vnet.ibm.com> --- include/linux/rculist.h | 23 +++++++++++++++++++++-- 1 file changed, 21 insertions(+), 2 deletions(-) diff --git a/include/linux/rculist.h b/include/linux/rculist.h index 4106721..45a0a9e 100644 --- a/include/linux/rculist.h +++ b/include/linux/rculist.h @@ -19,6 +19,21 @@ */ /* + * 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. -- 1.8.1.5 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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