netdev - Re: [PATCH v2] net: ks8851: Fix deadlock with the SPI chip variant

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Date: Sat, 6 Jul 2024 10:38:08 +0200
From: Ronald Wahl <ronald.wahl@...itan.com>
To: Jakub Kicinski <kuba@...nel.org>, Ronald Wahl <rwahl@....de>
Cc: "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
 Simon Horman <horms@...nel.org>, netdev@...r.kernel.org,
 stable@...r.kernel.org
Subject: Re: [PATCH v2] net: ks8851: Fix deadlock with the SPI chip variant

On 06.07.24 02:39, Jakub Kicinski wrote:
> On Thu,  4 Jul 2024 19:47:56 +0200 Ronald Wahl wrote:
>> --- a/drivers/net/ethernet/micrel/ks8851_spi.c
>> +++ b/drivers/net/ethernet/micrel/ks8851_spi.c
>> @@ -385,7 +385,7 @@ static netdev_tx_t ks8851_start_xmit_spi(struct sk_buff *skb,
>>      netif_dbg(ks, tx_queued, ks->netdev,
>>                "%s: skb %p, %d@%p\n", __func__, skb, skb->len, skb->data);
>>
>> -    spin_lock(&ks->statelock);
>> +    spin_lock_bh(&ks->statelock);
>>
>>      if (ks->queued_len + needed > ks->tx_space) {
>>              netif_stop_queue(dev);
>> @@ -395,7 +395,7 @@ static netdev_tx_t ks8851_start_xmit_spi(struct sk_buff *skb,
>>              skb_queue_tail(&ks->txq, skb);
>>      }
>>
>> -    spin_unlock(&ks->statelock);
>> +    spin_unlock_bh(&ks->statelock);
>
> this one probably can stay as spin_lock() since networking stack only
> calls xmit in BH context.

I already suspected this it was more a mental hint here. I will remove it.

> But I see 2 other spin_lock(statelock) in the
> driver which I'm not as sure about. Any taking of this lock has to be
> _bh() unless you're sure the caller is already in BH.

The other two instances are not in BH context as far as I know but also
do not interfere with BH. The one in ks8861_tx_work protects only
variable assignments used only inside the driver and the one in
ks8851_set_rx_mode also only does some driver local variable stuff and a
schedule_work which as far as I know has nothing to do with BH because
workqueues are running in process context. Am I wrong here?

- ron

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