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

[Ronald Wahl <ronald.wahl@raritan.com>]

On 06.07.24 10:38, Ronald Wahl wrote:
> 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?

I guess I found a misunderstanding on my side: I was assuming that a
softirq cannot asynchronously interrupt a spin lock protected section. Maybe
this is wrong. In the one place where I'm waking the queue again the
spin_lock_bh avoids synchronously triggering the BH processing while still
holding a spinlock.

I will use the _bh variants on the two other places.
  - ron

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