Subject: Re: [PATCH] bonding: 3ad: Fix the conflict between bond_update_slave_arr and the state machine

From: Jay Vosburgh <[email-masked]>
Date: Mon, 26 Apr 2021 12:29:22 -0700

David Miller <[email-masked]> wrote:

>From: Jay Vosburgh <[email-masked]>
>Date: Mon, 26 Apr 2021 08:22:37 -0700
>
>> David Miller <[email-masked]> wrote:
>> 
>>>From: jinyiting <[email-masked]>
>>>Date: Wed, 21 Apr 2021 16:38:21 +0800
>>>
>>>> The bond works in mode 4, and performs down/up operations on the bond
>>>> that is normally negotiated. The probability of bond-> slave_arr is NULL
>>>> 
>>>> Test commands:
>>>>    ifconfig bond1 down
>>>>    ifconfig bond1 up
>>>> 
>>>> The conflict occurs in the following process：
>>>> 
>>>> __dev_open (CPU A)
>>>> --bond_open
>>>>   --queue_delayed_work(bond->wq,&bond->ad_work,0);
>>>>   --bond_update_slave_arr
>>>>     --bond_3ad_get_active_agg_info
>>>> 
>>>> ad_work(CPU B)
>>>> --bond_3ad_state_machine_handler
>>>>   --ad_agg_selection_logic
>>>> 
>>>> ad_work runs on cpu B. In the function ad_agg_selection_logic, all
>>>> agg->is_active will be cleared. Before the new active aggregator is
>>>> selected on CPU B, bond_3ad_get_active_agg_info failed on CPU A,
>>>> bond->slave_arr will be set to NULL. The best aggregator in
>>>> ad_agg_selection_logic has not changed, no need to update slave arr.
>>>> 
>>>> The conflict occurred in that ad_agg_selection_logic clears
>>>> agg->is_active under mode_lock, but bond_open -> bond_update_slave_arr
>>>> is inspecting agg->is_active outside the lock.
>>>> 
>>>> Also, bond_update_slave_arr is normal for potential sleep when
>>>> allocating memory, so replace the WARN_ON with a call to might_sleep.
>>>> 
>>>> Signed-off-by: jinyiting <[email-masked]>
>>>> ---
>>>> 
>>>> Previous versions:
>>>>  * https://lore.kernel.org/netdev/[email-masked]/
>>>> 
>>>>  drivers/net/bonding/bond_main.c | 7 ++++---
>>>>  1 file changed, 4 insertions(+), 3 deletions(-)
>>>> 
>>>> diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
>>>> index 74cbbb2..83ef62d 100644
>>>> --- a/drivers/net/bonding/bond_main.c
>>>> +++ b/drivers/net/bonding/bond_main.c
>>>> @@ -4406,7 +4404,9 @@ int bond_update_slave_arr(struct bonding *bond, struct slave *skipslave)
>>>>  	if (BOND_MODE(bond) == BOND_MODE_8023AD) {
>>>>  		struct ad_info ad_info;
>>>>  
>>>> +		spin_lock_bh(&bond->mode_lock);
>>>
>>>The code paths that call this function with mode_lock held will now deadlock.
>> 
>> 	No path should be calling bond_update_slave_arr with mode_lock
>> already held (it expects RTNL only); did you find one?
>> 
>> 	My concern is that there's something else that does the opposite
>> order, i.e., mode_lock first, then RTNL, but I haven't found an example.
>> 
>
>This patch is removing a lockdep assertion masking sure that mode_lock was held
>when this function was called.  That should have been triggering all the time, right?

	The line in question is:
	
#ifdef CONFIG_LOCKDEP
	WARN_ON(lockdep_is_held(&bond->mode_lock));
#endif

	The WARN_ON is triggering if mode_lock is held, not asserting
that mode_lock is held.  I think that's wrong anyway, since mode_lock
could be held by some other thread, leading to false positives, thus the
change to might_sleep.

	-J

---
	-Jay Vosburgh, [email-masked]