netdev - Re: [PATCH net-next v3 15/16] net/mlx5e: take the rtnl lock when calling netif_set_xps

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Date:   Mon, 15 Mar 2021 16:53:02 +0200
From:   Maxim Mikityanskiy <maximmi@...dia.com>
To:     Antoine Tenart <atenart@...nel.org>,
        Saeed Mahameed <saeed@...nel.org>,
        Tariq Toukan <tariqt@...dia.com>, <alexander.duyck@...il.com>,
        <davem@...emloft.net>, <kuba@...nel.org>
CC:     <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next v3 15/16] net/mlx5e: take the rtnl lock when
 calling netif_set_xps_queue

On 2021-03-15 10:38, Antoine Tenart wrote:
> Quoting Saeed Mahameed (2021-03-12 21:54:18)
>> On Fri, 2021-03-12 at 16:04 +0100, Antoine Tenart wrote:
>>> netif_set_xps_queue must be called with the rtnl lock taken, and this
>>> is
>>> now enforced using ASSERT_RTNL(). mlx5e_attach_netdev was taking the
>>> lock conditionally, fix this by taking the rtnl lock all the time.
>>>
>>> Signed-off-by: Antoine Tenart <atenart@...nel.org>
>>> ---
>>>   drivers/net/ethernet/mellanox/mlx5/core/en_main.c | 11 +++--------
>>>   1 file changed, 3 insertions(+), 8 deletions(-)
>>>
>>> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
>>> b/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
>>> index ec2fcb2a2977..96cba86b9f0d 100644
>>> --- a/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
>>> +++ b/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
>>> @@ -5557,7 +5557,6 @@ static void mlx5e_update_features(struct
>>> net_device *netdev)
>>>   
>>>   int mlx5e_attach_netdev(struct mlx5e_priv *priv)
>>>   {
>>> -       const bool take_rtnl = priv->netdev->reg_state ==
>>> NETREG_REGISTERED;
>>>          const struct mlx5e_profile *profile = priv->profile;
>>>          int max_nch;
>>>          int err;
>>> @@ -5578,15 +5577,11 @@ int mlx5e_attach_netdev(struct mlx5e_priv
>>> *priv)
>>>           * 2. Set our default XPS cpumask.
>>>           * 3. Build the RQT.
>>>           *
>>> -        * rtnl_lock is required by netif_set_real_num_*_queues in case
>>> the
>>> -        * netdev has been registered by this point (if this function
>>> was called
>>> -        * in the reload or resume flow).
>>> +        * rtnl_lock is required by netif_set_xps_queue.
>>>           */
>>
>> There is a reason why it is conditional:
>> we had a bug in the past of double locking here:
>>
>> [ 4255.283960] echo/644 is trying to acquire lock:
>>
>>   [ 4255.285092] ffffffff85101f90 (rtnl_mutex){+..}, at:
>> mlx5e_attach_netdev0xd4/0×3d0 [mlx5_core]
>>
>>   [ 4255.287264]
>>
>>   [ 4255.287264] but task is already holding lock:
>>
>>   [ 4255.288971] ffffffff85101f90 (rtnl_mutex){+..}, at:
>> ipoib_vlan_add0×7c/0×2d0 [ib_ipoib]
>>
>> ipoib_vlan_add is called under rtnl and will eventually call
>> mlx5e_attach_netdev, we don't have much control over this in mlx5
>> driver since the rdma stack provides a per-prepared netdev to attach to
>> our hw. maybe it is time we had a nested rtnl lock ..
> 
> Thanks for the explanation. So as you said, we can't based the locking
> decision only on the driver own state / information...
> 
> What about `take_rtnl = !rtnl_is_locked();`?

It won't work, because the lock may be taken by some other unrelated 
thread. By doing `if (!rtnl_is_locked()) rtnl_lock()` we defeat the 
purpose of the lock, because we will proceed to the critical section 
even if we should wait until some other thread releases the lock.

> Thanks!
> Antoine
>