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

[Paul E. McKenney]

On Fri, Aug 23, 2013 at 02:08:22PM -0700, Paul E. McKenney wrote:
> On Fri, Aug 23, 2013 at 01:16:53PM -0400, Mathieu Desnoyers wrote:
> > * Paul E. McKenney (paulmck at linux.vnet.ibm.com) wrote:
> > > On Thu, Aug 22, 2013 at 09:33:18PM -0700, Stephen Hemminger wrote:

[ . . . ]

> > > > +
> > > > +/**
> > > > + * 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)
> 
> Or I need to fix this one as well.  ;-)

In that vein...  Is there anything like typeof() that also preserves
sparse's notion of address space?  Wrapping an ACCESS_ONCE() around
"p" in the assignment above results in sparse errors.

							Thanx, Paul

							Thanx, Paul