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Date:   Sat, 27 Jun 2020 11:55:21 -0700
From:   Cong Wang <>
To:     Sean Tranchetti <>
Cc:     David Miller <>,
        Linux Kernel Network Developers <>,
        Pravin B Shelar <>,
        Subash Abhinov Kasiviswanathan <>
Subject: Re: [PATCH net] genetlink: take netlink table lock when (un)registering

On Fri, Jun 26, 2020 at 5:32 PM Sean Tranchetti <> wrote:
> A potential deadlock can occur during registering or unregistering a new
> generic netlink family between the main nl_table_lock and the cb_lock where
> each thread wants the lock held by the other, as demonstrated below.
> 1) Thread 1 is performing a netlink_bind() operation on a socket. As part
>    of this call, it will call netlink_lock_table(), incrementing the
>    nl_table_users count to 1.
> 2) Thread 2 is registering (or unregistering) a genl_family via the
>    genl_(un)register_family() API. The cb_lock semaphore will be taken for
>    writing.
> 3) Thread 1 will call genl_bind() as part of the bind operation to handle
>    subscribing to GENL multicast groups at the request of the user. It will
>    attempt to take the cb_lock semaphore for reading, but it will fail and
>    be scheduled away, waiting for Thread 2 to finish the write.
> 4) Thread 2 will call netlink_table_grab() during the (un)registration
>    call. However, as Thread 1 has incremented nl_table_users, it will not
>    be able to proceed, and both threads will be stuck waiting for the
>    other.
> To avoid this scenario, the locks should be acquired in the same order by
> both threads. Since both the register and unregister functions need to take
> the nl_table_lock in their processing, it makes sense to explicitly acquire
> them before they lock the genl_mutex and the cb_lock. In unregistering, no
> other change is needed aside from this locking change.

Like the kernel test robot reported, you can not call genl_lock_all while
holding netlink_table_grab() which is effectively a write lock.

To me, it seems genl_bind() can be just removed as there is no one
in-tree uses family->mcast_bind(). Can you test the attached patch?
It seems sufficient to fix this deadlock.


View attachment "genetlink-mcast-bind.diff" of type "text/x-patch" (2716 bytes)

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