netdev - Re: [PATCH net] team: fix hang in team_mode

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Date:   Mon, 20 Apr 2020 20:18:13 +0900
From:   Taehee Yoo <ap420073@...il.com>
To:     Jiri Pirko <jiri@...nulli.us>
Cc:     David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net] team: fix hang in team_mode_get()

On Mon, 20 Apr 2020 at 19:21, Jiri Pirko <jiri@...nulli.us> wrote:
>

Hi Jiri,
Thank you for your review!

> Sat, Apr 18, 2020 at 06:17:29PM CEST, ap420073@...il.com wrote:
> >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@...il.com>
> >---
> > drivers/net/team/team.c | 10 +++++++++-
> > 1 file changed, 9 insertions(+), 1 deletion(-)
> >
> >diff --git a/drivers/net/team/team.c b/drivers/net/team/team.c
> >index 4004f98e50d9..21702bc23705 100644
> >--- a/drivers/net/team/team.c
> >+++ b/drivers/net/team/team.c
> >@@ -465,9 +465,15 @@ EXPORT_SYMBOL(team_mode_unregister);
> >
> > static const struct team_mode *team_mode_get(const char *kind)
> > {
> >-      struct team_mode_item *mitem;
> >       const struct team_mode *mode = NULL;
> >+      struct team_mode_item *mitem;
> >+      bool put = false;
> >
> >+#if IS_MODULE(CONFIG_NET_TEAM)
> >+      if (!try_module_get(THIS_MODULE))
>
> Can't you call this in case this is not a module? Wouldn't THIS_MODULE
> be NULL then? try_module_get() handles that correctly.
>
>
> >+              return NULL;
> >+      put = true;
> >+#endif
> >       spin_lock(&mode_list_lock);
> >       mitem = __find_mode(kind);
> >       if (!mitem) {
> >@@ -483,6 +489,8 @@ static const struct team_mode *team_mode_get(const char *kind)
> >       }
> >
> >       spin_unlock(&mode_list_lock);
> >+      if (put)
> >+              module_put(THIS_MODULE);
>
> Can't you just put this under the same "if IS_MODULE" statement and
> avoid the "put" variable? Or in case the statement is not needed, just
> do plain module_put call.
>
> Otherwise, the patch looks fine.
>

You're right, If the team is not a module, THIS_MODULE will be null.
try_module_get() and module_put() deal with NULL well.
So, I will send a v2 patch that will remove the #if statement and
the put variable.

Thanks a lot!
Taehee Yoo