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

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Date:   Wed, 24 Jan 2018 05:16:33 +0300
From:   Kirill Tkhai <ktkhai@...tuozzo.com>
To:     Stephen Hemminger <stephen@...workplumber.org>
Cc:     edumazet@...gle.com, netdev@...r.kernel.org, davem@...emloft.net,
        dsahern@...il.com, fw@...len.de, lucien.xin@...il.com,
        daniel@...earbox.net, mschiffer@...verse-factory.net,
        jakub.kicinski@...ronome.com, vyasevich@...il.com, jbenc@...hat.com
Subject: Re: [PATCH] net: Make synchronize_net() be expedited only when it's
 really need

On 23.01.2018 19:26, Stephen Hemminger wrote:
> On Mon, 22 Jan 2018 12:41:41 +0300
> Kirill Tkhai <ktkhai@...tuozzo.com> 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 <ktkhai@...tuozzo.com>
>> ---
>>  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.

If it's so, could you please point the place, where rtnl_is_locked()
actually breaks the AB BA deadlock you referred?

Thanks,
Kirill