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

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Message-ID: <161582121653.3996.16019131946329402786@kwain.local>
Date:   Mon, 15 Mar 2021 16:13:36 +0100
From:   Antoine Tenart <atenart@...nel.org>
To:     Maxim Mikityanskiy <maximmi@...dia.com>,
        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

Quoting Maxim Mikityanskiy (2021-03-15 15:53:02)
> On 2021-03-15 10:38, Antoine Tenart wrote:
> > Quoting Saeed Mahameed (2021-03-12 21:54:18)
> >> 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...