Re: [PATCH] net: fix possible deadlocks in rtnl_trylock/unlock

[Eric Dumazet <eric.dumazet@gmail.com>]

On Sat, 2012-12-01 at 18:29 +0100, Simon Wunderlich wrote:
> If rtnl_trylock() is used to prevent circular dependencies, rtnl_unlock()
> may destroy this attempt because it first unlocks rtnl_mutex but may
> lock it again later. The callgraph roughly looks like:
> 
> rtnl_unlock()
>    netdev_run_todo()
>       __rtnl_unlock()
>       netdev_wait_allrefs()
>          rtnl_lock()
>          ...
>          __rtnl_unlock()
> 
> There are two users which have possible deadlocks and are fixed in this
> patch: batman-adv and bridge. Their problematic access pattern is the same:
> 
> notifier_call handler:
>  * holds rtnl lock when called
>  * calls sysfs code at some point (acquiring sysfs locks)
> 
> sysfs code:
>  * holds sysfs lock when called
>  * calls rtnl_trylock() and rtnl_unlock(), rtnl_unlock() calls rtnl_lock
>    implicitly
> 
> Fix this by exporting __rtnl_unlock() to just unlock the mutex without
> implicitly locking again.
> 
> Reported-by: Sven Eckelmann <sven@narfation.org>
> Signed-off-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
> 
> ---
> Of course, an alternative would be to not lock again after unlocking
> within rtnl_unlock(), or put the todo handling into the locked section.
> I'm not familiar enough with this code to know what would be best.
> 
> There are others using rtnl_trylock(), but I'm not sure if they
> are affected.
> ---
>  net/batman-adv/sysfs.c   |    2 +-
>  net/bridge/br_sysfs_br.c |    2 +-
>  net/bridge/br_sysfs_if.c |    2 +-
>  net/core/rtnetlink.c     |    1 +
>  4 files changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/net/batman-adv/sysfs.c b/net/batman-adv/sysfs.c
> index 66518c7..41b74aa 100644
> --- a/net/batman-adv/sysfs.c
> +++ b/net/batman-adv/sysfs.c
> @@ -635,7 +635,7 @@ static ssize_t batadv_store_mesh_iface(struct kobject *kobj,
>  	ret = batadv_hardif_enable_interface(hard_iface, buff);
>  
>  unlock:
> -	rtnl_unlock();
> +	__rtnl_unlock();
>  out:
>  	batadv_hardif_free_ref(hard_iface);
>  	return ret;
> diff --git a/net/bridge/br_sysfs_br.c b/net/bridge/br_sysfs_br.c
> index c5c0593..c122782 100644
> --- a/net/bridge/br_sysfs_br.c
> +++ b/net/bridge/br_sysfs_br.c
> @@ -142,7 +142,7 @@ static ssize_t store_stp_state(struct device *d,
>  	if (!rtnl_trylock())
>  		return restart_syscall();
>  	br_stp_set_enabled(br, val);
> -	rtnl_unlock();
> +	__rtnl_unlock();
>  
>  	return len;
>  }

I have no idea of why you believe there is a problem here.

Could you explain how net_todo_list could be not empty ?

As long as no device is unregistered between
rtnl_trylock()/rtnl_unlock(), there is no possible deadlock.