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

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Date:   Mon, 20 Apr 2020 12:21:23 +0200
From:   Jiri Pirko <jiri@...nulli.us>
To:     Taehee Yoo <ap420073@...il.com>
Cc:     davem@...emloft.net, kuba@...nel.org, netdev@...r.kernel.org
Subject: Re: [PATCH net] team: fix hang in team_mode_get()

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.



> 	return mode;
> }
> 
>-- 
>2.17.1
>