linux-kernel - Re: [PATCH net 7/9] net/mlx5e: Properly access RCU protected qdisc

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Message-ID: <bdfe62f4-8b70-4284-b06d-e50ea6ae2d88@nvidia.com>
Date: Thu, 12 Jun 2025 10:31:45 +0300
From: Mark Bloch <mbloch@...dia.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: "David S. Miller" <davem@...emloft.net>, Paolo Abeni <pabeni@...hat.com>,
 Eric Dumazet <edumazet@...gle.com>, Andrew Lunn <andrew+netdev@...n.ch>,
 Simon Horman <horms@...nel.org>, saeedm@...dia.com, gal@...dia.com,
 leonro@...dia.com, tariqt@...dia.com, Leon Romanovsky <leon@...nel.org>,
 netdev@...r.kernel.org, linux-rdma@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH net 7/9] net/mlx5e: Properly access RCU protected
 qdisc_sleeping variable

On 12/06/2025 0:40, Jakub Kicinski wrote:
> On Tue, 10 Jun 2025 18:15:12 +0300 Mark Bloch wrote:
>> qdisc_sleeping variable is declared as "struct Qdisc __rcu" and
>> as such needs proper annotation while accessing it.
>>
>> Without rtnl_dereference(), the following error is generated by smatch:
> 
> sparse ?

Right, just tested it myself, it's indeed with sparse.

> 
>>
>> drivers/net/ethernet/mellanox/mlx5/core/en/qos.c:377:40: warning:
>>   incorrect type in initializer (different address spaces)
>> drivers/net/ethernet/mellanox/mlx5/core/en/qos.c:377:40:    expected
>>   struct Qdisc *qdisc
>> drivers/net/ethernet/mellanox/mlx5/core/en/qos.c:377:40:    got struct
>>   Qdisc [noderef] __rcu *qdisc_sleeping
>>
>> Fixes: 214baf22870c ("net/mlx5e: Support HTB offload")
> 
> I don't think this is a functional change? We don't treat silencing
> compiler warnings as fixes, not for sparse or W=1 warnings.

Well Eric's commit: d636fc5dd692c8f4e00ae6e0359c0eceeb5d9bdb
that added this annotation was because of a syzbot report.

Anyway, we don't mind pushing via net-next.

Mark

> 
>> Signed-off-by: Leon Romanovsky <leonro@...dia.com>
>> Reviewed-by: Tariq Toukan <tariqt@...dia.com>
>> Signed-off-by: Mark Bloch <mbloch@...dia.com>
>> ---
>>  drivers/net/ethernet/mellanox/mlx5/core/en/qos.c | 4 +++-
>>  1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en/qos.c b/drivers/net/ethernet/mellanox/mlx5/core/en/qos.c
>> index f0744a45db92..2f32111210f8 100644
>> --- a/drivers/net/ethernet/mellanox/mlx5/core/en/qos.c
>> +++ b/drivers/net/ethernet/mellanox/mlx5/core/en/qos.c
>> @@ -374,7 +374,9 @@ void mlx5e_reactivate_qos_sq(struct mlx5e_priv *priv, u16 qid, struct netdev_que
>>  void mlx5e_reset_qdisc(struct net_device *dev, u16 qid)
>>  {
>>  	struct netdev_queue *dev_queue = netdev_get_tx_queue(dev, qid);
>> -	struct Qdisc *qdisc = dev_queue->qdisc_sleeping;
>> +	struct Qdisc *qdisc;
>> +
>> +	qdisc = rtnl_dereference(dev_queue->qdisc_sleeping);
>>  
>>  	if (!qdisc)
> 
> nit: no new line between action and error check