[lttng-dev] [RFC] adding into middle of RCU list

[Mathieu Desnoyers]

* Paul E. McKenney (paulmck at 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 <stephen at 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 <paulmck at 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.
> 

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com