Date:   Tue, 23 Jan 2018 07:45:31 -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 7:29 AM, Kirill Tkhai <ktkhai@...tuozzo.com> wrote:
> On 23.01.2018 18:12, Eric Dumazet wrote:
>> 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.
>
> Not missed. I grepped all over the kernel source, and this is how BUGs
> in staging drivers were found. I just can't believe we really need
> this check. Ok, then how about something like this:
>
> int rtnl_is_locked(void)
> {
> #ifdef CONFIG_DEBUG_KERNEL
>         BUG_ON(!in_task());
> #endif
>         return __mutex_owner(&rtnl_mutex) == current;
> }
>
> CONFIG_DEBUG_KERNEL is because of rtnl_is_locked() is used widely,
> and the check has only the debug purpose.
>

So it looks you want to fix 3 bugs in staging, by changing
rtnl_is_locked() semantic.
This semantic had no recent changes (for last 10 years at least)

I am fine with such a change but for net-next tree.
We are too late in linux-4.15 for such a change.

For net tree, please independently fix the staging bugs, that is less
controversial.

Thank you.

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