netdev - Re: [net,PATCH] net: ks8851: Queue RX packets in IRQ handler instead of disabling BHs

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Date: Tue, 30 Apr 2024 16:58:46 +0200
From: Marek Vasut <marex@...x.de>
To: Ronald Wahl <ronald.wahl@...itan.com>, netdev@...r.kernel.org
Cc: "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>
Subject: Re: [net,PATCH] net: ks8851: Queue RX packets in IRQ handler instead
 of disabling BHs

On 4/30/24 10:24 AM, Ronald Wahl wrote:
> On 30.04.24 03:14, Marek Vasut wrote:
>> Currently the driver uses local_bh_disable()/local_bh_enable() in its
>> IRQ handler to avoid triggering net_rx_action() softirq on exit from
>> netif_rx(). The net_rx_action() could trigger this driver .start_xmit
>> callback, which is protected by the same lock as the IRQ handler, so
>> calling the .start_xmit from netif_rx() from the IRQ handler critical
>> section protected by the lock could lead to an attempt to claim the
>> already claimed lock, and a hang.
>>
>> The local_bh_disable()/local_bh_enable() approach works only in case
>> the IRQ handler is protected by a spinlock, but does not work if the
>> IRQ handler is protected by mutex, i.e. this works for KS8851 with
>> Parallel bus interface, but not for KS8851 with SPI bus interface.
>>
>> Remove the BH manipulation and instead of calling netif_rx() inside
>> the IRQ handler code protected by the lock, queue all the received
>> SKBs in the IRQ handler into a queue first, and once the IRQ handler
>> exits the critical section protected by the lock, dequeue all the
>> queued SKBs and push them all into netif_rx(). At this point, it is
>> safe to trigger the net_rx_action() softirq, since the netif_rx()
>> call is outside of the lock that protects the IRQ handler.
>>
>> Fixes: be0384bf599c ("net: ks8851: Handle softirqs at the end of IRQ 
>> thread to fix hang")
>> Signed-off-by: Marek Vasut <marex@...x.de>
>> ---
>> Cc: "David S. Miller" <davem@...emloft.net>
>> Cc: Eric Dumazet <edumazet@...gle.com>
>> Cc: Jakub Kicinski <kuba@...nel.org>
>> Cc: Paolo Abeni <pabeni@...hat.com>
>> Cc: Ronald Wahl <ronald.wahl@...itan.com>
>> Cc: Simon Horman <horms@...nel.org>
>> Cc: netdev@...r.kernel.org
> 
> To me the code looks good. An iperf3 test shows that it now has even
> slightly more throughput in my setup (two interconnected ks8851-spi).
> Thanks for this fix!
> 
> Tested-by: Ronald Wahl <ronald.wahl@...itan.com>

That's nice. Thank you for testing. Sorry for the breakage.