Subject: Re: [PATCH] net: Make synchronize_net() be expedited only when it's really need

From: Stephen Hemminger <[email-masked]>
Date: Tue, 23 Jan 2018 08:26:38 -0800

On Mon, 22 Jan 2018 12:41:41 +0300
Kirill Tkhai <[email-masked]> wrote:

> Commit be3fc413da9e "net: use synchronize_rcu_expedited()" introducing
> synchronize_net() says:
> 
>     >When we hold RTNL mutex, we would like to spend some cpu cycles but not
>     >block too long other processes waiting for this mutex.
>     >We also want to setup/dismantle network features as fast as possible at
>     >boot/shutdown time.
>     >This patch makes synchronize_net() call the expedited version if RTNL is
>     >locked.  
> 
> At the time of the commit (May 23 2011) there was no possible to differ,
> who is the actual owner of the mutex. Only the fact that it's locked
> by someone at the moment. So (I guess) this is the only reason the generic
> primitive mutex_is_locked() was used.
> 
> But now mutex owner is available outside the locking subsystem and
> __mutex_owner() may be used instead (there is an example in audit_log_start()).
> So, let's make expensive synchronize_rcu_expedited() be used only
> when a caller really owns rtnl_mutex().
> 
> There are several possibilities to fix that. The first one is
> to fix synchronize_net(), the second is to change rtnl_is_locked().
> 
> I prefer the second, as it seems it's more intuitive for people
> to think that rtnl_is_locked() is about current process, not
> about the fact mutex is locked in general. Grep over kernel
> sources just proves this fact:
> 
> drivers/staging/rtl8723bs/os_dep/osdep_service.c:297
> drivers/staging/rtl8723bs/os_dep/osdep_service.c:316
> 
>         if (!rtnl_is_locked())
>                 ret = register_netdev(pnetdev);
>         else
>                 ret = register_netdevice(pnetdev);

Conditional locking like this especially in the registration path
is wrong and broken. The driver should just be removed for this.

> drivers/staging/wilc1000/linux_mon.c:310
> 
> 	if (rtnl_is_locked()) {
> 		rtnl_unlock();
> 		rollback_lock = true;
> 	}

This driver like most of staging is sh*t.

> Side effect of this patch is three BUGs in above examples
> become fixed.
> 
> Signed-off-by: Kirill Tkhai <[email-masked]>
> ---
>  net/core/rtnetlink.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
> index 16d644a4f974..a5ddf373ffa9 100644
> --- a/net/core/rtnetlink.c
> +++ b/net/core/rtnetlink.c
> @@ -117,7 +117,7 @@ EXPORT_SYMBOL(rtnl_trylock);
>  
>  int rtnl_is_locked(void)
>  {
> -	return mutex_is_locked(&rtnl_mutex);
> +	return __mutex_owner(&rtnl_mutex) == current;
>  }
>  EXPORT_SYMBOL(rtnl_is_locked);

No. You are breaking the concept of is_locked. It means the mutex
is locked anywhere, not just in the calling process. For example,
rtnl_is_locked is used to break AB BA lock deadlock in sysfs.