Re: [PATCH net] net: mld: do not use system_wq in the mld

[Taehee Yoo <ap420073@gmail.com>]

Hi Eric,
Thank you so much for your review!

On 7/21/22 23:04, Eric Dumazet wrote:
 > On Thu, Jul 21, 2022 at 2:03 PM Taehee Yoo <ap420073@gmail.com> wrote:
 >>
 >> mld works are supposed to be executed in mld_wq.
 >> But mld_{query | report}_work() calls schedule_delayed_work().
 >> schedule_delayed_work() internally uses system_wq.
 >> So, this would cause the reference count leak.
 >
 > I do not think the changelog is accurate.
 > At least I do not understand it yet.
 >
 > We can not unload the ipv6 module, so destroy_workqueue(mld_wq) is 
never used.
 >
 >
 >
 >>
 >> splat looks like:
 >>   unregister_netdevice: waiting for br1 to become free. Usage count = 2
 >>   leaked reference.
 >>    ipv6_add_dev+0x3a5/0x1070
 >>    addrconf_notify+0x4f3/0x1760
 >>    notifier_call_chain+0x9e/0x180
 >>    register_netdevice+0xd10/0x11e0
 >>    br_dev_newlink+0x27/0x100 [bridge]
 >>    __rtnl_newlink+0xd85/0x14e0
 >>    rtnl_newlink+0x5f/0x90
 >>    rtnetlink_rcv_msg+0x335/0x9a0
 >>    netlink_rcv_skb+0x121/0x350
 >>    netlink_unicast+0x439/0x710
 >>    netlink_sendmsg+0x75f/0xc00
 >>    ____sys_sendmsg+0x694/0x860
 >>    ___sys_sendmsg+0xe9/0x160
 >>    __sys_sendmsg+0xbe/0x150
 >>    do_syscall_64+0x3b/0x90
 >>    entry_SYSCALL_64_after_hwframe+0x63/0xcd
 >>
 >> Fixes: f185de28d9ae ("mld: add new workqueues for process mld events")
 >> Signed-off-by: Taehee Yoo <ap420073@gmail.com>
 >> ---
 >>   net/ipv6/mcast.c | 14 ++++++++------
 >>   1 file changed, 8 insertions(+), 6 deletions(-)
 >>
 >> diff --git a/net/ipv6/mcast.c b/net/ipv6/mcast.c
 >> index 7f695c39d9a8..87c699d57b36 100644
 >> --- a/net/ipv6/mcast.c
 >> +++ b/net/ipv6/mcast.c
 >> @@ -1522,7 +1522,6 @@ static void mld_query_work(struct work_struct 
*work)
 >>
 >>                  if (++cnt >= MLD_MAX_QUEUE) {
 >>                          rework = true;
 >> -                       schedule_delayed_work(&idev->mc_query_work, 0);
 >>                          break;
 >>                  }
 >>          }
 >> @@ -1533,8 +1532,10 @@ static void mld_query_work(struct work_struct 
*work)
 >>                  __mld_query_work(skb);
 >>          mutex_unlock(&idev->mc_lock);
 >>
 >> -       if (!rework)
 >> -               in6_dev_put(idev);
 >> +       if (rework && queue_delayed_work(mld_wq, 
&idev->mc_query_work, 0))
 >
 > It seems the 'real issue' was that
 > schedule_delayed_work(&idev->mc_query_work, 0) could be a NOP
 > because the work queue was already scheduled ?
 >

I think your assumption is right.
I tested the below scenario, which occurs the real issue.
THREAD0                            THREAD1
mld_report_work()
                                    spin_lock_bh()
                                    if (!mod_delayed_work()) <-- queued
                                            in6_dev_hold();
                                    spin_unlock_bh()
spin_lock_bh()
schedule_delayed_work() <-- return false, already queued by THREAD1
spin_unlock_bh()
return;
//no in6_dev_put() regardless return value of schedule_delayed_work().

In order to check, I added printk like below.
         if (++cnt >= MLD_MAX_QUEUE) { 

                 rework = true; 

                 if (!schedule_delayed_work(&idev->mc_report_work, 0))
                         printk("[TEST]%s %u \n", __func__, __LINE__);
                 break; 


If the TEST log message is printed, work is already queued by other logic.
So, it indicates a reference count is leaked.
The result is that I can see log messages only when the reference count 
leak occurs.
So, although I tested it only for 1 hour, I'm sure that this bug comes 
from missing check a return value of schedule_delayed_work().

As you said, this changelog is not correct.
system_wq and mld_wq are not related to this issue.

I would like to send a v2 patch after some more tests.
The v2 patch will change the commit message.

 >
 >
 >> +               return;
 >> +
 >> +       in6_dev_put(idev);
 >>   }
 >>
 >>   /* called with rcu_read_lock() */
 >> @@ -1624,7 +1625,6 @@ static void mld_report_work(struct work_struct 
*work)
 >>
 >>                  if (++cnt >= MLD_MAX_QUEUE) {
 >>                          rework = true;
 >> -                       schedule_delayed_work(&idev->mc_report_work, 0);
 >>                          break;
 >>                  }
 >>          }
 >> @@ -1635,8 +1635,10 @@ static void mld_report_work(struct 
work_struct *work)
 >>                  __mld_report_work(skb);
 >>          mutex_unlock(&idev->mc_lock);
 >>
 >> -       if (!rework)
 >> -               in6_dev_put(idev);
 >> +       if (rework && queue_delayed_work(mld_wq, 
&idev->mc_report_work, 0))
 >> +               return;
 >> +
 >> +       in6_dev_put(idev);
 >>   }
 >>
 >>   static bool is_in(struct ifmcaddr6 *pmc, struct ip6_sf_list *psf, 
int type,
 >> --
 >> 2.17.1
 >>

Thanks a lot!
Taehee Yoo