netdev - Re: [net,PATCH v3] net: ks8851: Queue RX packets in IRQ handler instead of disabling BHs

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Message-ID: <8de19188-577e-4e74-89c9-82755d0423fa@denx.de>
Date: Fri, 3 May 2024 22:03:34 +0200
From: Marek Vasut <marex@...x.de>
To: Eric Dumazet <edumazet@...gle.com>
Cc: netdev@...r.kernel.org, Ronald Wahl <ronald.wahl@...itan.com>,
 "David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>
Subject: Re: [net,PATCH v3] net: ks8851: Queue RX packets in IRQ handler
 instead of disabling BHs

On 5/3/24 9:08 AM, Eric Dumazet wrote:
> On Thu, May 2, 2024 at 8:34 PM Marek Vasut <marex@...x.de> 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")
>> Tested-by: Ronald Wahl <ronald.wahl@...itan.com> # KS8851 SPI
>> Signed-off-by: Marek Vasut <marex@...x.de>
> 
> Reviewed-by: Eric Dumazet <edumazet@...gle.com>

Thank you and Jakub for your help with this.