linux-kernel - Re: [PATCH net-next 05/16] idpf: fix Rx descriptor ready check barrier in splitq

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Message-ID: <b1255dff-91c4-4042-bb3d-78245065c669@intel.com>
Date: Wed, 12 Mar 2025 18:10:05 +0100
From: Alexander Lobakin <aleksander.lobakin@...el.com>
To: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
CC: <intel-wired-lan@...ts.osuosl.org>, Michal Kubiak
	<michal.kubiak@...el.com>, Tony Nguyen <anthony.l.nguyen@...el.com>, "Przemek
 Kitszel" <przemyslaw.kitszel@...el.com>, Andrew Lunn <andrew+netdev@...n.ch>,
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, "Alexei
 Starovoitov" <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
	"Jesper Dangaard Brouer" <hawk@...nel.org>, John Fastabend
	<john.fastabend@...il.com>, Simon Horman <horms@...nel.org>,
	<bpf@...r.kernel.org>, <netdev@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next 05/16] idpf: fix Rx descriptor ready check
 barrier in splitq

From: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
Date: Fri, 7 Mar 2025 11:17:11 +0100

> On Wed, Mar 05, 2025 at 05:21:21PM +0100, Alexander Lobakin wrote:
>> No idea what the current barrier position was meant for. At that point,
>> nothing is read from the descriptor, only the pointer to the actual one
>> is fetched.
>> The correct barrier usage here is after the generation check, so that
>> only the first qword is read if the descriptor is not yet ready and we
>> need to stop polling. Debatable on coherent DMA as the Rx descriptor
>> size is <= cacheline size, but anyway, the current barrier position
>> only makes the codegen worse.
> 
> Makes sense:
> Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
> 
> But you know the process... :P fixes should go to -net.

The thing's that it makes no difference for regular skb Rx, but with
ret != XDP_PASS it starts making issues. So yes, this is a fix, but
I don't think it should go separately.

> 
>>
>> Fixes: 3a8845af66ed ("idpf: add RX splitq napi poll support")
>> Signed-off-by: Alexander Lobakin <aleksander.lobakin@...el.com>
>> ---
>>  drivers/net/ethernet/intel/idpf/idpf_txrx.c | 8 ++------
>>  1 file changed, 2 insertions(+), 6 deletions(-)

Thanks,
Olek