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@xxxxxxxxxxxxxxxxxx) 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
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