Re: [PATCH net] packet: call fanout_release, while UNREGISTERING a netdev

[Anoob Soman]

On 31/01/17 18:00, Eric Dumazet wrote:
On Tue, 2017-01-31 at 17:03 +0000, Anoob Soman wrote:
On 30/01/17 19:44, Eric Dumazet wrote:
On Mon, 2017-01-30 at 19:08 +0000, Anoob Soman wrote:
On 30/01/17 17:26, Eric Dumazet wrote:
On Thu, 2016-10-06 at 20:50 -0400, David Miller wrote:
From: Anoob Soman <anoob.so...@citrix.com>
Date: Wed, 5 Oct 2016 15:12:54 +0100

If a socket has FANOUT sockopt set, a new proto_hook is registered
as part of fanout_add(). When processing a NETDEV_UNREGISTER event in
af_packet, __fanout_unlink is called for all sockets, but prot_hook which was
registered as part of fanout_add is not removed. Call fanout_release, on a
NETDEV_UNREGISTER, which removes prot_hook and removes fanout from the
fanout_list.

This fixes BUG_ON(!list_empty(&dev->ptype_specific)) in netdev_run_todo()

Signed-off-by: Anoob Soman <anoob.so...@citrix.com>
Applied and queued up for -stable, thanks.
This commit (6664498280cf "packet: call fanout_release, while
UNREGISTERING a netdev")
looks buggy :

We end up calling fanout_release() while holding a spinlock
( spin_lock(&po->bind_lock); )

But fanout_release() grabs a mutex ( mutex_lock(&fanout_mutex) ), and
this is absolutely not valid while holding a spinlock.
Yes, that is wrong.

Anoob, can you cook a fix, I guess you have a way to reproduce the thing
that wanted a kernel patch ?

(Please build your test kernel with CONFIG_LOCKDEP=y)
Sure, I am planning to move fanout_release(sk) after
spin_unlock(bind_lock). Something like this.
                                   }
                                   if (msg == NETDEV_UNREGISTER) {
                                           packet_cached_dev_reset(po);
-                                       fanout_release(sk);
                                           po->ifindex = -1;
                                           if (po->prot_hook.dev)
dev_put(po->prot_hook.dev);
                                           po->prot_hook.dev = NULL;
                                   }
                                   spin_unlock(&po->bind_lock);
+                               if (msg == NETDEV_UNREGISTER) {
+                                       fanout_release(sk);
+                               }
                           }
                           break;

I will quickly test it out.
It wont be enough.

You need to also fix a race if two cpus call fanout_release(sk) at the
same time.

Thanks.



Hi Eric,

I have ran into some problem trying to enable CONFIG_LOCKDEP. I think
this particular scenario, taking mutex_lock() while holding a spin_lock
debugging, requires CONFIG_DEBUG_ATOMIC_SLEEP to be enabled.
CONFIG_DEBUG_ATOMIC_SLEEP, selects CONFIG_PREEMPT_COUNT and my kernel
doesn't behave well if PREEMPTION is enabled. I am trying to reproduce
this issue in a way that I might be able to use debug_atomic_sleep.

Meanwhile, I have modified patch fix the race.

So you can definitely have in a .config all these at the same time
(LOCKDEP,  non PREEMPT, and DEBUG_ATOMIC_SLEEP)




$ egrep "DEBUG_ATOMIC_SLEEP|PREEMPT|LOCKDEP" .config
CONFIG_LOCKDEP_SUPPORT=y
CONFIG_PREEMPT_NOTIFIERS=y
CONFIG_PREEMPT_NONE=y
# CONFIG_PREEMPT_VOLUNTARY is not set
# CONFIG_PREEMPT is not set
CONFIG_PREEMPT_COUNT=y
CONFIG_LOCKDEP=y
# CONFIG_DEBUG_LOCKDEP is not set
CONFIG_DEBUG_ATOMIC_SLEEP=y


yes, thats exactly what I have.

$ egrep "DEBUG_ATOMIC_SLEEP|PREEMPT|LOCKDEP" .config
CONFIG_LOCKDEP_SUPPORT=y
# CONFIG_PREEMPT_NONE is not set
CONFIG_PREEMPT_VOLUNTARY=y
# CONFIG_PREEMPT is not set
CONFIG_PREEMPT_COUNT=y
CONFIG_LOCKDEP=y
# CONFIG_DEBUG_LOCKDEP is not set
CONFIG_DEBUG_ATOMIC_SLEEP=y

I initially thought CONFIG_PREEMPT_COUNT enables CONFIG_PREEMPT, but 
looks like all it does is to inc/dec preempt_count.

Let me give the test a spin again, and see why everything seems to fall 
apart.