On Fri, 23 Aug 2013 13:16:53 -0400 Mathieu Desnoyers <mathieu.desnoy...@efficios.com> wrote:
> * Paul E. McKenney (paul...@linux.vnet.ibm.com) wrote: > > On Thu, Aug 22, 2013 at 09:33:18PM -0700, Stephen Hemminger wrote: > > > I needed to add into the middle of an RCU list, does this make sense. > > > > > > > > > > > > From a45892b0d49ac5fe449ba7e19c646cb17f7cee57 Mon Sep 17 00:00:00 2001 > > > From: Stephen Hemminger <step...@networkplumber.org> > > > Date: Thu, 22 Aug 2013 21:27:04 -0700 > > > Subject: [PATCH] Add list_splice_init_rcu to allow insertion into a RCU > > > list > > > > > > Simplified version of the version in kernel. > > > --- > > > urcu/rculist.h | 32 ++++++++++++++++++++++++++++++++ > > > 1 file changed, 32 insertions(+) > > > > > > diff --git a/urcu/rculist.h b/urcu/rculist.h > > > index 1fd2df3..2e8a5a0 100644 > > > --- a/urcu/rculist.h > > > +++ b/urcu/rculist.h > > > @@ -72,6 +72,38 @@ void cds_list_del_rcu(struct cds_list_head *elem) > > > CMM_STORE_SHARED(elem->prev->next, elem->next); > > > } > > > > > > + > > > +/** > > > + * Splice an RCU-protected list into an existing list. > > > + * > > > + * Note that this function blocks in synchronize_rcu() > > > + * > > > + * Important note: this function is not called concurrently > > > + * with other updates to the list. > > > + */ > > > +static inline void caa_list_splice_init_rcu(struct cds_list_head *list, > > > + struct cds_list_head *head) > > > +{ > > > + struct cds_list_head *first = list->next; > > > + struct cds_list_head *last = list->prev; > > > + struct cds_list_head *at = head->next; > > > + > > > + if (cds_list_empty(list)) > > > + return; > > > + > > > + /* "first" and "last" tracking list, so initialize it. */ > > > + CDS_INIT_LIST_HEAD(list); > > > > This change is happening in the presence of readers on the list, right? > > For this to work reliably in the presence of mischievous compilers, > > wouldn't CDS_INIT_LIST_HEAD() need to use CMM_ACCESS_ONCE() for its > > pointer accesses? > > Actually, we have rcu_assign_pointer()/rcu_set_pointer() exactly for > this. They even skip the memory barrier if they store a NULL pointer. > > > > > Hmmm... The kernel version seems to have the same issue... > > The compiler memory model of the Linux kernel AFAIK does not require an > ACCESS_ONCE() for stores to word-aligned, word-sized integers/pointers, > even if those are expected to be read concurrently. For reference, see: > > #define __rcu_assign_pointer(p, v, space) \ > do { \ > smp_wmb(); \ > (p) = (typeof(*v) __force space *)(v); \ > } while (0) > > In userspace RCU, we require to match CMM_LOAD_SHARED() with > CMM_STORE_SHARED() (which are used by > rcu_dereference()/rcu_{set,assign}_pointer) whenever we concurrently > access a variable shared between threads. > > So I recommend using rcu_set_pointer() in userspace RCU, but I don't > think your patch is needed for Linux, given the Linux kernel compiler > memory model that is less strict than userspace RCU's model. > > Thanks, > > Mathieu > > > > Patch below, FWIW. > > > > Thanx, Paul > > > > > + > > > + /* Wait for any readers to finish using the list before splicing */ > > > + synchronize_rcu(); > > > + > > > + /* Readers are finished with the source list, so perform splice. */ > > > + last->next = at; > > > + rcu_assign_pointer(head->next, first); > > > + first->prev = head; > > > + at->prev = last; > > > +} > > > + > > > /* > > > * Iteration through all elements of the list must be done while > > > rcu_read_lock() > > > * is held. > > > -- > > > 1.7.10.4 > > > > 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 <paul...@linux.vnet.ibm.com> > > > > 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. > > > For what I need, it is probably simpler to do "insert in middle" rather than a full splice, so I am checking how to do that. _______________________________________________ lttng-dev mailing list lttng-dev@lists.lttng.org http://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev