On Sun, Apr 28, 2019 at 12:51 AM Jason Wang [off-list ref] wrote:

quoted

tun_net_xmit() doesn't have the chance to
access the change because it holding the rcu_read_lock().

The problem is the following codes:


         --tun->numqueues;

         ...

         synchronize_net();

We need make sure the decrement of tun->numqueues be visible to readers
after synchronize_net(). And in tun_net_xmit():

It doesn't matter at all. Readers are okay to read it even they still use the
stale tun->numqueues, as long as the tfile is not freed readers can read
whatever they want...

The decrement of tun->numqueues is just how we unpublish the old
tfile, it is still valid for readers to read it _after_ unpublish, we only need
to worry about free, not about unpublish. This is the whole spirit of RCU.

You need to rethink about my SOCK_RCU_FREE patch.

Thanks.