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

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Date:   Tue, 23 Jan 2018 07:12:21 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     Kirill Tkhai <ktkhai@...tuozzo.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        netdev <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        David Ahern <dsahern@...il.com>,
        Florian Westphal <fw@...len.de>,
        Xin Long <lucien.xin@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        mschiffer@...verse-factory.net, jakub.kicinski@...ronome.com,
        Vladislav Yasevich <vyasevich@...il.com>,
        Jiri Benc <jbenc@...hat.com>
Subject: Re: [PATCH] net: Make synchronize_net() be expedited only when it's
 really need

On Tue, Jan 23, 2018 at 6:41 AM, Kirill Tkhai <ktkhai@...tuozzo.com> wrote:
> 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.

You have not touched only this path, but all paths using ASSERT_RTNL()

This is why I think your patch would target net-next, not net tree.

> 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.

I believe you missed ASSERT_RTNL(), used all over the place.