Date:   Tue, 23 Jan 2018 08:05:31 -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 7:57 AM, Kirill Tkhai <ktkhai@...tuozzo.com> wrote:
> On 23.01.2018 18:45, Eric Dumazet wrote:
>> 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)
>
> No, I don't care about the staging. I care about excess actions
> (interrupts), that synchronize_rcu_expedited() sends. I wrote about
> that in patch description :)

This behavior is years old. What is suddenly the concern here ?

If we are concerned about expedited stuff, why do we allow it in the
first place ?

Please provide numbers, experiments, bugs caused by this expedited thing,
because right now I have no idea what problem you really have.

You are mixing several things in your patch attempt, and this is very confusing.



>
>> I am fine with such a change but for net-next tree.
>> We are too late in linux-4.15 for such a change.
>
> Thanks for your review again. Could you, please, clarify, which change is
> OK for you relatively to net-next: 1)w/o BUG_ON() or 2)with BUG_ON().
> Sorry for that I ask, but I hadn't understand, which change you mean :(

I mean that I see nothing urgent needing a change in rtnl_is_locked()
in net tree.

Now, if you need a temporary new rtnl_is_locked_by_me() I would not be
against that.
(to address staging bugs)

(See sock_owned_by_me() for one example...)

>
>> For net tree, please independently fix the staging bugs, that is less
>> controversial
>
> Since I had no the staging devices, I'll report to their maintainers
> after we found the final decision.
>
> Thanks,
> Kirill

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