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SubjectRe: unregister_netdevice: waiting for DEV to become free (2)
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This bug is the top crasher for syzbot and thus we want to fix. I need your
response regarding commit caacf05e5ad1abf0 ("ipv4: Properly purge netdev
references on uncached routes.") why you chose "a loopback device in that
namespace".

On 2019/04/16 23:00, Tetsuo Handa wrote:
> Hello, David S. Miller.
>
> I have a question regarding rt_flush_dev() introduced by commit caacf05e5ad1abf0
> ("ipv4: Properly purge netdev references on uncached routes.") which went to
> Linux 3.6-rc1. That commit started replacing "a device to unregister" with
> "a loopback device in that namespace", but there is no description why that
> commit chose "a loopback device in that namespace". If a device to unregister
> is "a loopback device in that namespace" itself, rt_flush_dev() becomes a no-op
> because dev == net->loopback_dev from the beginning. Apart from a problem that
> usage count keeps increasing because dev_put(rt->dst.dev) is not called after
> rt->dst.dev was replaced with a loopback device, replacing "a device to unregister"
> with "a loopback device in init namespace" (like shown below) avoids this problem.
>
> ----------
> diff --git a/net/ipv4/route.c b/net/ipv4/route.c
> index a5da63e..aff6a44 100644
> --- a/net/ipv4/route.c
> +++ b/net/ipv4/route.c
> @@ -1492,7 +1492,6 @@ static void ipv4_dst_destroy(struct dst_entry *dst)
>
> void rt_flush_dev(struct net_device *dev)
> {
> - struct net *net = dev_net(dev);
> struct rtable *rt;
> int cpu;
>
> @@ -1503,7 +1502,7 @@ void rt_flush_dev(struct net_device *dev)
> list_for_each_entry(rt, &ul->head, rt_uncached) {
> if (rt->dst.dev != dev)
> continue;
> - rt->dst.dev = net->loopback_dev;
> + rt->dst.dev = init_net.loopback_dev;
> dev_hold(rt->dst.dev);
> dev_put(dev);
> }
> ----------
>
> On 2019/04/15 22:36, Tetsuo Handa wrote:
>> On 2018/08/20 21:55, Julian Anastasov wrote:
>>> Well, only IPVS and tun in the game? But IPVS does not
>>> take any dev references for sync threads. Can it be a problem
>>> in tun? For example, a side effects from dst_cache_reset?
>>> May be dst_release is called too late? Here is what should happen
>>> on unregistration:
>>>
>>> - NETDEV_UNREGISTER event: rt_flush_dev changes dst->dev with lo
>>> but dst is not released
>>>
>>> - ndo_uninit/ip_tunnel_uninit: dst_cache_reset is called which
>>> does nothing!?! May be dst_release call is needed here.
>>>
>>> - no more references are expected here ...
>>>
>>> - netdev_run_todo -> netdev_wait_allrefs: loop here due to refcnt!=0
>>>
>>> - dev->priv_destructor (ip_tunnel_dev_free) calls dst_cache_destroy
>>> where dst_release is used but it is not reached because we loop in
>>> netdev_wait_allrefs above
>>>
>>> - dst_cache_destroy: really call dst_release
>>>
>>> In fact, after calling rt_flush_dev and replacing the
>>> dst->dev we should reach dev->priv_destructor (ip_tunnel_dev_free)
>>> for tun device where dst_release for lo should be called. But may be
>>> something prevents it, exit batching?
>>
>> I traced using debug printk() patch shown below.
>>
>> ----------------------------------------
>> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
>> index 26f69cf763f4..25f7acacf457 100644
>> --- a/include/linux/netdevice.h
>> +++ b/include/linux/netdevice.h
>> @@ -3726,6 +3726,11 @@ void netdev_run_todo(void);
>> */
>> static inline void dev_put(struct net_device *dev)
>> {
>> + if (!strcmp(dev->name, "lo")) {
>> + int count = netdev_refcnt_read(dev);
>> + printk("dev_put(%p): %u->%u\n", dev, count, count - 1);
>> + dump_stack();
>> + }
>> this_cpu_dec(*dev->pcpu_refcnt);
>> }
>>
>> @@ -3737,6 +3742,11 @@ static inline void dev_put(struct net_device *dev)
>> */
>> static inline void dev_hold(struct net_device *dev)
>> {
>> + if (!strcmp(dev->name, "lo")) {
>> + int count = netdev_refcnt_read(dev);
>> + printk("dev_hold(%p): %u->%u\n", dev, count, count + 1);
>> + dump_stack();
>> + }
>> this_cpu_inc(*dev->pcpu_refcnt);
>> }
>>
>> diff --git a/net/core/dev.c b/net/core/dev.c
>> index fdcff29df915..53ff4385c8f7 100644
>> --- a/net/core/dev.c
>> +++ b/net/core/dev.c
>> @@ -8897,9 +8897,9 @@ static void netdev_wait_allrefs(struct net_device *dev)
>>
>> refcnt = netdev_refcnt_read(dev);
>>
>> - if (time_after(jiffies, warning_time + 10 * HZ)) {
>> - pr_emerg("unregister_netdevice: waiting for %s to become free. Usage count = %d\n",
>> - dev->name, refcnt);
>> + if (time_after(jiffies, warning_time + HZ)) {
>> + pr_emerg("unregister_netdevice: waiting for %s to become free. Usage count = %d%s (%p)\n",
>> + dev->name, refcnt, netdev_reg_state(dev), dev);
>> warning_time = jiffies;
>> }
>> }
>> diff --git a/net/ipv4/route.c b/net/ipv4/route.c
>> index a5da63e5faa2..4c5baca105ed 100644
>> --- a/net/ipv4/route.c
>> +++ b/net/ipv4/route.c
>> @@ -1503,6 +1503,7 @@ void rt_flush_dev(struct net_device *dev)
>> list_for_each_entry(rt, &ul->head, rt_uncached) {
>> if (rt->dst.dev != dev)
>> continue;
>> + printk("rt_flush_dev(%p)->(%p)\n", dev, net->loopback_dev);
>> rt->dst.dev = net->loopback_dev;
>> dev_hold(rt->dst.dev);
>> dev_put(dev);
>> @@ -2560,6 +2561,7 @@ struct dst_entry *ipv4_blackhole_route(struct net *net, struct dst_entry *dst_or
>> new->input = dst_discard;
>> new->output = dst_discard_out;
>>
>> + printk("ipv4_blackhole_route(%p)->(%p)\n", new->dev, net->loopback_dev);
>> new->dev = net->loopback_dev;
>> if (new->dev)
>> dev_hold(new->dev);
>> ----------------------------------------
>>
>> When a new loopback device in a new network namespace is created using unshare(),
>> nothing is wrong.
>>
>> [ 60.873014][ T7306] dev_hold(00000000d9f4ea20): 0->1
>> [ 60.873019][ T7306] CPU: 4 PID: 7306 Comm: a.out Not tainted 5.1.0-rc5+ #177
>> [ 60.873021][ T7306] Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00 04/13/2018
>> [ 60.873022][ T7306] Call Trace:
>> [ 60.873031][ T7306] dump_stack+0xaa/0xd8
>> [ 60.873037][ T7306] net_rx_queue_update_kobjects+0x1f5/0x200
>> [ 60.873042][ T7306] netdev_register_kobject+0xf8/0x1a0
>> [ 60.873047][ T7306] register_netdevice+0x4cc/0x650
>> [ 60.873052][ T7306] register_netdev+0x23/0x40
>> [ 60.873056][ T7306] loopback_net_init+0x50/0xc0
>> [ 60.873059][ T7306] ? loopback_dev_init+0xa0/0xa0
>> [ 60.873064][ T7306] ops_init+0x4f/0x140
>> [ 60.873068][ T7306] setup_net+0xe7/0x250
>> [ 60.873072][ T7306] copy_net_ns+0xee/0x1e0
>> [ 60.873077][ T7306] create_new_namespaces+0x141/0x2a0
>> [ 60.873081][ T7306] unshare_nsproxy_namespaces+0x7e/0xf0
>> [ 60.873086][ T7306] ksys_unshare+0x268/0x4b0
>> [ 60.873090][ T7306] __x64_sys_unshare+0x16/0x20
>> [ 60.873095][ T7306] do_syscall_64+0x7c/0x180
>> [ 60.873099][ T7306] entry_SYSCALL_64_after_hwframe+0x44/0xa9
>>
>> But when some device in a network namespace calls rt_flush_dev(),
>> it gets a usage count on loopback device in that network namespace.
>>
>> [ 71.388104][ T7620] rt_flush_dev(00000000cd35e96a)->(00000000d9f4ea20)
>> [ 71.391757][ T7620] dev_hold(00000000d9f4ea20): 7->8
>> [ 71.394725][ T7620] CPU: 4 PID: 7620 Comm: a.out Not tainted 5.1.0-rc5+ #177
>> [ 71.398094][ T7620] Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00 04/13/2018
>> [ 71.403711][ T7620] Call Trace:
>> [ 71.405912][ T7620] dump_stack+0xaa/0xd8
>> [ 71.408252][ T7620] rt_flush_dev+0x177/0x1b0
>> [ 71.410802][ T7620] fib_netdev_event+0x150/0x1b0
>> [ 71.413270][ T7620] notifier_call_chain+0x47/0xd0
>> [ 71.415849][ T7620] raw_notifier_call_chain+0x2d/0x40
>> [ 71.418491][ T7620] ? tun_show_group+0x90/0x90
>> [ 71.421108][ T7620] call_netdevice_notifiers_info+0x32/0x70
>> [ 71.423854][ T7620] rollback_registered_many+0x421/0x680
>> [ 71.426583][ T7620] rollback_registered+0x68/0xb0
>> [ 71.429244][ T7620] unregister_netdevice_queue+0xa5/0x100
>> [ 71.432191][ T7620] __tun_detach+0x576/0x590
>> [ 71.435533][ T7620] tun_chr_close+0x41/0x80
>> [ 71.437957][ T7620] ? __tun_detach+0x590/0x590
>> [ 71.440500][ T7620] __fput+0xeb/0x2d0
>> [ 71.442816][ T7620] ____fput+0x15/0x20
>> [ 71.445090][ T7620] task_work_run+0xa9/0xd0
>> [ 71.447467][ T7620] do_exit+0x37a/0xf40
>> [ 71.449623][ T7620] do_group_exit+0x57/0xe0
>> [ 71.451826][ T7620] get_signal+0x114/0x950
>> [ 71.453989][ T7620] do_signal+0x2f/0x700
>> [ 71.456126][ T7620] ? handle_mm_fault+0x1a8/0x360
>> [ 71.458323][ T7620] ? __x64_sys_futex+0x179/0x210
>> [ 71.460620][ T7620] exit_to_usermode_loop+0x159/0x180
>> [ 71.462956][ T7620] do_syscall_64+0x15d/0x180
>> [ 71.465110][ T7620] entry_SYSCALL_64_after_hwframe+0x44/0xa9
>>
>> The usage count printed by "unregister_netdevice: waiting for lo to become free."
>> seems to match number of rt_flush_dev() traces shown above. (Complete log is at
>> http://I-love.SAKURA.ne.jp/tmp/serial-20190415.txt.xz .)
>>
>> Although netdev_wait_allrefs() is periodically calling
>> call_netdevice_notifiers(NETDEV_UNREGISTER, dev) in order to try to drop
>> the usgae count,
>>
>> list_for_each_entry(rt, &ul->head, rt_uncached) {
>> if (rt->dst.dev != dev)
>> continue;
>> rt->dst.dev = net->loopback_dev;
>> dev_hold(rt->dst.dev);
>> dev_put(dev);
>> }
>>
>> in rt_flush_dev() becomes a no-op because dev == net->loopback_dev and
>> therefore cannot drop the usage count forever. That is, netdev_wait_allrefs()
>> on a loopback device cannot make forward progress.
>>
>> [ 95.502947][ T4478] unregister_netdevice: waiting for lo to become free. Usage count = 28 (unregistered) (00000000d9f4ea20)
>> [ 95.509108][ T4478] rt_flush_dev(00000000d9f4ea20)->(00000000d9f4ea20)
>> [ 95.512598][ T4478] dev_hold(00000000d9f4ea20): 28->29
>> [ 95.517241][ T4478] CPU: 5 PID: 4478 Comm: kworker/u128:28 Not tainted 5.1.0-rc5+ #177
>> [ 95.522984][ T4478] Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00 04/13/2018
>> [ 95.532898][ T4478] Workqueue: netns cleanup_net
>> [ 95.537134][ T4478] Call Trace:
>> [ 95.539751][ T4478] dump_stack+0xaa/0xd8
>> [ 95.543642][ T4478] rt_flush_dev+0x177/0x1b0
>> [ 95.546835][ T4478] fib_netdev_event+0x150/0x1b0
>> [ 95.550609][ T4478] notifier_call_chain+0x47/0xd0
>> [ 95.557990][ T4478] raw_notifier_call_chain+0x2d/0x40
>> [ 95.563900][ T4478] call_netdevice_notifiers_info+0x32/0x70
>> [ 95.568655][ T4478] netdev_run_todo+0x197/0x410
>> [ 95.572554][ T4478] rtnl_unlock+0xe/0x10
>> [ 95.576836][ T4478] default_device_exit_batch+0x1ab/0x1d0
>> [ 95.579650][ T4478] ? do_wait_intr_irq+0xb0/0xb0
>> [ 95.582236][ T4478] ? unregister_netdevice_many+0x30/0x30
>> [ 95.584766][ T4478] ? dev_change_net_namespace+0x4e0/0x4e0
>> [ 95.587397][ T4478] ops_exit_list.isra.6+0x75/0x90
>> [ 95.590063][ T4478] cleanup_net+0x20d/0x380
>> [ 95.592373][ T4478] process_one_work+0x202/0x4f0
>> [ 95.595017][ T4478] worker_thread+0x3c/0x4b0
>> [ 95.597520][ T4478] kthread+0x139/0x160
>> [ 95.599822][ T4478] ? process_one_work+0x4f0/0x4f0
>> [ 95.602351][ T4478] ? kthread_destroy_worker+0x70/0x70
>> [ 95.604901][ T4478] ret_from_fork+0x35/0x40
>> [ 95.607249][ T4478] dev_put(00000000d9f4ea20): 29->28
>> [ 95.609935][ T4478] CPU: 5 PID: 4478 Comm: kworker/u128:28 Not tainted 5.1.0-rc5+ #177
>> [ 95.613282][ T4478] Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00 04/13/2018
>> [ 95.618699][ T4478] Workqueue: netns cleanup_net
>> [ 95.621345][ T4478] Call Trace:
>> [ 95.623381][ T4478] dump_stack+0xaa/0xd8
>> [ 95.625787][ T4478] rt_flush_dev+0x19f/0x1b0
>> [ 95.628284][ T4478] fib_netdev_event+0x150/0x1b0
>> [ 95.630827][ T4478] notifier_call_chain+0x47/0xd0
>> [ 95.633153][ T4478] raw_notifier_call_chain+0x2d/0x40
>> [ 95.635702][ T4478] call_netdevice_notifiers_info+0x32/0x70
>> [ 95.638372][ T4478] netdev_run_todo+0x197/0x410
>> [ 95.640641][ T4478] rtnl_unlock+0xe/0x10
>> [ 95.642828][ T4478] default_device_exit_batch+0x1ab/0x1d0
>> [ 95.645230][ T4478] ? do_wait_intr_irq+0xb0/0xb0
>> [ 95.647613][ T4478] ? unregister_netdevice_many+0x30/0x30
>> [ 95.650148][ T4478] ? dev_change_net_namespace+0x4e0/0x4e0
>> [ 95.652594][ T4478] ops_exit_list.isra.6+0x75/0x90
>> [ 95.654975][ T4478] cleanup_net+0x20d/0x380
>> [ 95.657140][ T4478] process_one_work+0x202/0x4f0
>> [ 95.659829][ T4478] worker_thread+0x3c/0x4b0
>> [ 95.662098][ T4478] kthread+0x139/0x160
>> [ 95.664119][ T4478] ? process_one_work+0x4f0/0x4f0
>> [ 95.666467][ T4478] ? kthread_destroy_worker+0x70/0x70
>> [ 95.668916][ T4478] ret_from_fork+0x35/0x40
>>
>> If we do something like
>>
>> list_for_each_entry(rt, &ul->head, rt_uncached) {
>> if (rt->dst.dev != dev)
>> continue;
>> - rt->dst.dev = net->loopback_dev;
>> + if (dev == net->loopback_dev)
>> + rt->dst.dev = init_net.loopback_dev;
>> + else
>> + rt->dst.dev = net->loopback_dev;
>> dev_hold(rt->dst.dev);
>> dev_put(dev);
>> }
>>
>> at rt_flush_dev(), I guess that this problem will go away. I don't
>> know which device should be used instead of init_net.loopback_dev .
>> But I guess that we need to somehow avoid getting usage count on
>> a loopback device (by e.g. using a dummy device which is not under
>> unregistration procedure) when we want to unregister that loopback
>> device.
>>
>

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