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    Subject[PATCH 5.6 061/167] team: fix hang in team_mode_get()
    Date
    From: Taehee Yoo <ap420073@gmail.com>

    [ Upstream commit 1c30fbc76b8f0c07c92a8ca4cd7c456612e17eb5 ]

    When team mode is changed or set, the team_mode_get() is called to check
    whether the mode module is inserted or not. If the mode module is not
    inserted, it calls the request_module().
    In the request_module(), it creates a child process, which is
    the "modprobe" process and waits for the done of the child process.
    At this point, the following locks were used.
    down_read(&cb_lock()); by genl_rcv()
    genl_lock(); by genl_rcv_msc()
    rtnl_lock(); by team_nl_cmd_options_set()
    mutex_lock(&team->lock); by team_nl_team_get()

    Concurrently, the team module could be removed by rmmod or "modprobe -r"
    The __exit function of team module is team_module_exit(), which calls
    team_nl_fini() and it tries to acquire following locks.
    down_write(&cb_lock);
    genl_lock();
    Because of the genl_lock() and cb_lock, this process can't be finished
    earlier than request_module() routine.

    The problem secenario.
    CPU0 CPU1
    team_mode_get
    request_module()
    modprobe -r team_mode_roundrobin
    team <--(B)
    modprobe team <--(A)
    team_mode_roundrobin

    By request_module(), the "modprobe team_mode_roundrobin" command
    will be executed. At this point, the modprobe process will decide
    that the team module should be inserted before team_mode_roundrobin.
    Because the team module is being removed.

    By the module infrastructure, the same module insert/remove operations
    can't be executed concurrently.
    So, (A) waits for (B) but (B) also waits for (A) because of locks.
    So that the hang occurs at this point.

    Test commands:
    while :
    do
    teamd -d &
    killall teamd &
    modprobe -rv team_mode_roundrobin &
    done

    The approach of this patch is to hold the reference count of the team
    module if the team module is compiled as a module. If the reference count
    of the team module is not zero while request_module() is being called,
    the team module will not be removed at that moment.
    So that the above scenario could not occur.

    Fixes: 3d249d4ca7d0 ("net: introduce ethernet teaming device")
    Signed-off-by: Taehee Yoo <ap420073@gmail.com>
    Reviewed-by: Jiri Pirko <jiri@mellanox.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    drivers/net/team/team.c | 4 ++++
    1 file changed, 4 insertions(+)

    --- a/drivers/net/team/team.c
    +++ b/drivers/net/team/team.c
    @@ -468,6 +468,9 @@ static const struct team_mode *team_mode
    struct team_mode_item *mitem;
    const struct team_mode *mode = NULL;

    + if (!try_module_get(THIS_MODULE))
    + return NULL;
    +
    spin_lock(&mode_list_lock);
    mitem = __find_mode(kind);
    if (!mitem) {
    @@ -483,6 +486,7 @@ static const struct team_mode *team_mode
    }

    spin_unlock(&mode_list_lock);
    + module_put(THIS_MODULE);
    return mode;
    }


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