Peter Zijlstra <pet...@infradead.org> wrote:
> On Mon, Oct 23, 2017 at 05:32:00PM +0200, Florian Westphal wrote:
>
> > > 1) it not in fact a refcount, so using refcount_t is silly
> >
> > Your suggestion is...?
>
> Normal atomic_t
Why? refcount_t gives debug options to catch leaks/underflows,
atomic_t does not.
Is refcount_t only supposed to be used with dec_and_test patterns?
> To avoid the problem of te inc being observed late.
>
> > However, this refcount_dec is misplaced anyway as it would need
> > to occur from nlcb->done() (the handler function gets stored in socket for
> > use by next recvmsg), so this change is indeed not helpful at all.
> >
> > > 3) waiting with a schedule()/yield() loop is complete crap and subject
> > > life-locks, imagine doing that rtnl_unregister_all() from a RT task.
>
> > Alternatively we can of course sleep instead of schedule() but that
> > doesn't appear too appealing either (albeit it is a lot less intrusive).
>
> That is much better than a yield loop.
>
> > Any other idea?
>
> This rtnetlink_rcv_msg() is called from softirq-context, right? Also,
> all that stuff happens with rcu_read_lock() held.
No, its called from process context.
I need to run now but plan to test and submit something like this:
diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
--- a/net/core/rtnetlink.c
+++ b/net/core/rtnetlink.c
@@ -263,7 +263,7 @@ void rtnl_unregister_all(int protocol)
synchronize_net();
while (refcount_read(&rtnl_msg_handlers_ref[protocol]) > 1)
- schedule();
+ msleep(1);
kfree(handlers);
}
EXPORT_SYMBOL_GPL(rtnl_unregister_all);
@@ -4149,6 +4149,16 @@ static int rtnl_stats_dump(struct sk_buff *skb, struct
netlink_callback *cb)
return skb->len;
}
+
+static int rtnl_dumper_done(struct netlink_callback *cb)
+{
+ unsigned int family = (unsigned long)cb->data;
+
+ refcount_dec(&rtnl_msg_handlers_ref[family]);
+ smp_mb__after_atomic();
+ return 0;
+}
+
/* Process one rtnetlink message. */
static int rtnetlink_rcv_msg(struct sk_buff *skb, struct nlmsghdr *nlh,
@@ -4207,6 +4217,7 @@ static int rtnetlink_rcv_msg(struct sk_buff *skb, struct
nlmsghdr *nlh,
}
refcount_inc(&rtnl_msg_handlers_ref[family]);
+ smp_mb__after_atomic();
if (type == RTM_GETLINK - RTM_BASE)
min_dump_alloc = rtnl_calcit(skb, nlh);
@@ -4217,11 +4228,12 @@ static int rtnetlink_rcv_msg(struct sk_buff *skb,
struct nlmsghdr *nlh,
{
struct netlink_dump_control c = {
.dump = dumpit,
+ .done = rtnl_dumper_done,
.min_dump_alloc = min_dump_alloc,
+ .data = (void *)(unsigned long)family,
};
err = netlink_dump_start(rtnl, skb, nlh, &c);
}
- refcount_dec(&rtnl_msg_handlers_ref[family]);
return err;
}
