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

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Date:   Tue, 23 Jan 2018 17:41:26 +0300
From:   Kirill Tkhai <ktkhai@...tuozzo.com>
To:     Eric Dumazet <eric.dumazet@...il.com>, edumazet@...gle.com,
        netdev@...r.kernel.org, davem@...emloft.net
Cc:     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

Hi, Eric,

thanks for your review. 

On 22.01.2018 20:15, Eric Dumazet wrote:
> On Mon, 2018-01-22 at 12:41 +0300, Kirill Tkhai 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);
>>
>> drivers/staging/wilc1000/linux_mon.c:310
>>
>> 	if (rtnl_is_locked()) {
>> 		rtnl_unlock();
>> 		rollback_lock = true;
>> 	}
>>
>> 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);
>>  
>>
> 
> Seems good to me, but this looks a net-next candidate to me.

No objections. What for this may be need for net tree?! Only to fix
the staging drivers above. But AFAIR, staging drivers guarantees, which
the kernel gives, are that they may be compiled. If so, we do not need
this in net tree.

> Note that this does not catch illegal uses from BH, where current is
> not related to our context of execution.

It's true, but the patch is about reducing of synchronize_rcu_expedited()
calls. There was no an objective to limit area of the places, where
rtnl_is_locked() can be used. For me it looks like another logical change.
If we really need that, one more patch on top of this may be submitted.
But honestly, I can't imagine someone really needs that check.

Thanks,
Kirill