On Tue, Jan 23, 2018 at 6:41 AM, Kirill Tkhai <ktk...@virtuozzo.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 <ktk...@virtuozzo.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.
