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

Quoting Maxim Mikityanskiy (2021-03-15 15:53:02)

On 2021-03-15 10:38, Antoine Tenart wrote:

quoted

Quoting Saeed Mahameed (2021-03-12 21:54:18)

quoted

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.

Ah, that's right...