Re: [PATCH 08/10] virtio: console: add locks around buffer removal in port unplug path

[Rusty Russell <rusty@rustcorp.com.au>]

Amit Shah <amit.shah@redhat.com> writes:
> On (Mon) 22 Jul 2013 [15:26:22], Rusty Russell wrote:
>> Amit Shah <amit.shah@redhat.com> writes:
>> > The removal functions act on the vqs, and the vq operations need to be
>> > locked.
>> >
>> > Signed-off-by: Amit Shah <amit.shah@redhat.com>
>> 
>> How can userspace access the port now?  By the time we are cleaning up
>> buffers, there should be no possibility of such accesses.
>
> close(), can happen when the port is being unplugged.  We're just
> making sure here that port_fops_release() and unplug_port() don't try
> to free up the same data at the same time.

Why doesn't reference counting help us here?  Surely the last one should
clean up?

>> The number of bugfixes here is deeply disturbing.
>
> Yes, the first three fix a bug - close() after unplug.  However, the
> others are inadequate locking fixes which I noticed while fixing that
> bug.
>
> Port unplug isn't a frequently-used or tested path, so these were
> lying unnoticed so far.
>
>>  I wonder if it's be
>> easier to rewrite it all with a lock per port, and one global to protect
>> ports_driver_data.
>
> Hm, with this series, I don't see anything that might need extra
> locking.  Though I'll take a look at this afresh in a while -- and see
> if we could simplify something.
>
> Given that this was necessary only for unplug operations, (and they
> aren't a 'regular operation', so we could drop the stable@ for the
> series?), are you OK with this series for now?

Let's skip stable@ for theoretical bugs.  Please repost.

Cheers,
Rusty.