linux-kernel - Re: [PATCH net 1/2] net: ethernet: oa_tc6: fix infinite loop error when tx credits becomes 0

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Message-ID: <b20ed55e-cb77-4821-9b5f-37cdf3d01df5@intel.com>
Date: Wed, 20 Nov 2024 11:54:41 -0800
From: Jacob Keller <jacob.e.keller@...el.com>
To: Parthiban Veerasooran <parthiban.veerasooran@...rochip.com>,
	<davem@...emloft.net>, <edumazet@...gle.com>, <kuba@...nel.org>,
	<pabeni@...hat.com>, <horms@...nel.org>, <saeedm@...dia.com>,
	<anthony.l.nguyen@...el.com>, <netdev@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <andrew@...n.ch>, <corbet@....net>,
	<linux-doc@...r.kernel.org>, <robh+dt@...nel.org>,
	<krzysztof.kozlowski+dt@...aro.org>, <conor+dt@...nel.org>,
	<devicetree@...r.kernel.org>, <horatiu.vultur@...rochip.com>,
	<ruanjinjie@...wei.com>, <steen.hegelund@...rochip.com>,
	<vladimir.oltean@....com>
CC: <masahiroy@...nel.org>, <alexanderduyck@...com>, <krzk+dt@...nel.org>,
	<robh@...nel.org>, <rdunlap@...radead.org>, <hkallweit1@...il.com>,
	<linux@...linux.org.uk>, <UNGLinuxDriver@...rochip.com>,
	<Thorsten.Kummermehr@...rochip.com>, <Pier.Beruto@...emi.com>,
	<Selvamani.Rajagopal@...emi.com>, <Nicolas.Ferre@...rochip.com>,
	<benjamin.bigler@...nformulastudent.ch>, <linux@...ler.io>,
	<markku.vorne@...power.com>
Subject: Re: [PATCH net 1/2] net: ethernet: oa_tc6: fix infinite loop error
 when tx credits becomes 0



On 11/20/2024 5:51 AM, Parthiban Veerasooran wrote:
> SPI thread wakes up to perform SPI transfer whenever there is an TX skb
> from n/w stack or interrupt from MAC-PHY. Ethernet frame from TX skb is
> transferred based on the availability tx credits in the MAC-PHY which is
> reported from the previous SPI transfer. Sometimes there is a possibility
> that TX skb is available to transmit but there is no tx credits from
> MAC-PHY. In this case, there will not be any SPI transfer but the thread
> will be running in an endless loop until tx credits available again.
> 
> So checking the availability of tx credits along with TX skb will prevent
> the above infinite loop. When the tx credits available again that will be
> notified through interrupt which will trigger the SPI transfer to get the
> available tx credits.
> 
> Fixes: 53fbde8ab21e ("net: ethernet: oa_tc6: implement transmit path to transfer tx ethernet frames")
> Signed-off-by: Parthiban Veerasooran <parthiban.veerasooran@...rochip.com>
> ---
>  drivers/net/ethernet/oa_tc6.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/ethernet/oa_tc6.c b/drivers/net/ethernet/oa_tc6.c
> index f9c0dcd965c2..4c8b0ca922b7 100644
> --- a/drivers/net/ethernet/oa_tc6.c
> +++ b/drivers/net/ethernet/oa_tc6.c
> @@ -1111,8 +1111,9 @@ static int oa_tc6_spi_thread_handler(void *data)
>  		/* This kthread will be waken up if there is a tx skb or mac-phy
>  		 * interrupt to perform spi transfer with tx chunks.
>  		 */
> -		wait_event_interruptible(tc6->spi_wq, tc6->waiting_tx_skb ||
> -					 tc6->int_flag ||
> +		wait_event_interruptible(tc6->spi_wq, tc6->int_flag ||
> +					 (tc6->waiting_tx_skb &&
> +					 tc6->tx_credits) ||
>  					 kthread_should_stop());
>  

Ok, so previously we check:

waiting_tx_skb || int_flag

Now we check:

int_flag || (waiting_tx_skb && tx_credits) || kthread_should_stop.

We didn't check kthread_should_stop before and this isn't mentioned in
the commit message, (or at least its not clear to me).

Whats the purpose behind that? I guess you want to wake up immediately
when kthread_should_stop() so that we can shutdown the kthread ASAP? Is
the condition "waiting_tx_skb && tx_credits" such that we might
otherwise not wake up, but with just "waiting_tx_skb" we definitely wake
up and stop earlier?

I think that change makes sense but I don't like that it was not called
out in the commit message.

The code seems correct to me otherwise.

>  		if (kthread_should_stop())