Date:   Tue, 23 Jan 2018 18:57:54 +0300
From:   Kirill Tkhai <ktkhai@...tuozzo.com>
To:     Eric Dumazet <edumazet@...gle.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 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 :)

> 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 :(

> 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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