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

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Message-ID: <1485885650.6360.122.camel@edumazet-glaptop3.roam.corp.google.com>
Date:   Tue, 31 Jan 2017 10:00:50 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Anoob Soman <anoob.soman@...rix.com>
Cc:     David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH net] packet: call fanout_release, while UNREGISTERING a
 netdev

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.soman@...rix.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.soman@...rix.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