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)

quoted

On Fri, 2021-03-12 at 16:04 +0100, Antoine Tenart wrote:

quoted

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@kernel.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