On Tue, Feb 10, 2015 at 09:03:50AM -0500, Peter Hurley wrote:
> On 02/06/2015 09:08 PM, Mathieu Desnoyers wrote:
> > A lockless_dereference() appears to be missing in llist_del_first().
> > It should only matter for Alpha in practice.
> >
> > Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> > CC: Huang Ying <ying.huang@xxxxxxxxx>
> > CC: Paul McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > CC: David Howells <dhowells@xxxxxxxxxx>
> > CC: Pranith Kumar <bobby.prani@xxxxxxxxx>
> > CC: stable@xxxxxxxxxxxxxxx # v3.1+
> > ---
> > lib/llist.c | 8 +++++++-
> > 1 file changed, 7 insertions(+), 1 deletion(-)
> >
> > diff --git a/lib/llist.c b/lib/llist.c
> > index f76196d..f34e176 100644
> > --- a/lib/llist.c
> > +++ b/lib/llist.c
> > @@ -26,6 +26,7 @@
> > #include <linux/export.h>
> > #include <linux/interrupt.h>
> > #include <linux/llist.h>
> > +#include <linux/rcupdate.h>
>
> Pranith,
>
> I didn't realize you put lockless_dereference() in rcupdate.h
>
> If the point of lockless_reference() is to provide a utility function for
> situations _not_ involving RCU, then it doesn't make sense to provide it
> in an RCU header file.
OK, I'll bite. Just where do you suggest putting it? ;-)
That question aside, lockless_dereference() does resemble the
rcu_dereference() family of APIs. This of course means that having it in
rcupdate.h near rcu_dereference() makes it easier to maintain, given that
needed changes to one are likely to require at least review of the rest.
Thanx, Paul
> Regards,
> Peter Hurley
>
> PS - That's not an objection to this patch, though.
>
> > /**
> > @@ -67,7 +68,12 @@ struct llist_node *llist_del_first(struct llist_head *head)
> > {
> > struct llist_node *entry, *old_entry, *next;
> >
> > - entry = head->first;
> > + /*
> > + * Load entry before entry->next. Matches the implicit
> > + * memory barrier before the cmpxchg in llist_add_batch(),
> > + * which ensures entry->next is stored before entry.
> > + */
> > + entry = lockless_dereference(head->first);
> > for (;;) {
> > if (entry == NULL)
> > return NULL;
> >
>
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