Re: [take21 1/4] kevent: Core files.

[Eric Dumazet]

Evgeniy Polyakov a e'crit :
> On Sat, Oct 28, 2006 at 03:23:40PM +0200, Eric Dumazet (dada1@xxxxxxxxxxxxx) wrote:
> > > diff --git a/kernel/kevent/kevent_user.c b/kernel/kevent/kevent_user.c
> > > index 711a8a8..ecee668 100644
> > > --- a/kernel/kevent/kevent_user.c
> > > +++ b/kernel/kevent/kevent_user.c
> > > @@ -235,6 +235,36 @@ static void kevent_free_rcu(struct rcu_h
> > > }
> > >
> > > /*
> > > + * Must be called under u->ready_lock.
> > > + * This function removes kevent from ready queue and 
> > > + * tries to add new kevent into ring buffer.
> > > + */
> > > +static void kevent_remove_ready(struct kevent *k)
> > > +{
> > > +	struct kevent_user *u = k->user;
> > > +
> > > +	list_del(&k->ready_entry);
> > Arg... no
> >
> > You cannot call list_del() , then check overflow_kevent.
> >
> > I you call list_del on what happens to be the kevent pointed by 
> > overflow_kevent, you loose...
>
> This function is always called from appropriate context, where it is
> guaranteed that it is safe to call list_del:
> 1. when kevent is removed. It is called after check, that given kevent 
> is in the ready queue.
> 2. when dequeued from ready queue, which means that it can be removed
> from that queue.

Could you please check the list_del() function ?

file include/linux/list.h

static inline void list_del(struct list_head *entry)
{
  __list_del(entry->prev, entry->next);
  entry->next = LIST_POISON1;
  entry->prev = LIST_POISON2;
}

So, after calling list_del(&k->read_entry);
next and prev are basically destroyed.

So when you write later :

+        if (!err || u->overflow_kevent == k) {
+            if (u->overflow_kevent->ready_entry.next == &u->ready_list)
+                u->overflow_kevent = NULL;
+            else
+                u->overflow_kevent = + 
list_entry(u->overflow_kevent->ready_entry.next, + 
struct kevent, ready_entry);
+        }


then you have a problem, since

list_entry(k->ready_entry.next, struct kevent, ready_entry);

will give you garbage.

Eric
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