linux-kernel - Re: [Intel-wired-lan] [PATCH net v3] igc: Prevent garbled TX queue with XDP ZEROCOPY

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Message-ID: <68dcdf3c-226c-9a94-0860-68c6d4b8ca0e@linux.intel.com>
Date:   Mon, 17 Jul 2023 09:32:33 +0300
From:   "naamax.meir" <naamax.meir@...ux.intel.com>
To:     Florian Kauer <florian.kauer@...utronix.de>,
        Jesse Brandeburg <jesse.brandeburg@...el.com>,
        Tony Nguyen <anthony.l.nguyen@...el.com>,
        "David S . Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Vedang Patel <vedang.patel@...el.com>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        Jithu Joseph <jithu.joseph@...el.com>,
        Andre Guedes <andre.guedes@...el.com>,
        Simon Horman <simon.horman@...igine.com>
Cc:     netdev@...r.kernel.org, kurt@...utronix.de,
        linux-kernel@...r.kernel.org, intel-wired-lan@...ts.osuosl.org
Subject: Re: [Intel-wired-lan] [PATCH net v3] igc: Prevent garbled TX queue
 with XDP ZEROCOPY

On 6/30/2023 15:03, Florian Kauer wrote:
> In normal operation, each populated queue item has
> next_to_watch pointing to the last TX desc of the packet,
> while each cleaned item has it set to 0. In particular,
> next_to_use that points to the next (necessarily clean)
> item to use has next_to_watch set to 0.
> 
> When the TX queue is used both by an application using
> AF_XDP with ZEROCOPY as well as a second non-XDP application
> generating high traffic, the queue pointers can get in
> an invalid state where next_to_use points to an item
> where next_to_watch is NOT set to 0.
> 
> However, the implementation assumes at several places
> that this is never the case, so if it does hold,
> bad things happen. In particular, within the loop inside
> of igc_clean_tx_irq(), next_to_clean can overtake next_to_use.
> Finally, this prevents any further transmission via
> this queue and it never gets unblocked or signaled.
> Secondly, if the queue is in this garbled state,
> the inner loop of igc_clean_tx_ring() will never terminate,
> completely hogging a CPU core.
> 
> The reason is that igc_xdp_xmit_zc() reads next_to_use
> before acquiring the lock, and writing it back
> (potentially unmodified) later. If it got modified
> before locking, the outdated next_to_use is written
> pointing to an item that was already used elsewhere
> (and thus next_to_watch got written).
> 
> Fixes: 9acf59a752d4 ("igc: Enable TX via AF_XDP zero-copy")
> Signed-off-by: Florian Kauer <florian.kauer@...utronix.de>
> Reviewed-by: Kurt Kanzenbach <kurt@...utronix.de>
> Tested-by: Kurt Kanzenbach <kurt@...utronix.de>
> Acked-by: Vinicius Costa Gomes <vinicius.gomes@...el.com>
> ---
> 
> v2 -> v3:
> Resolve merge conflict
> 
> v1 -> v2:
> I added some more context for further clarification,
> but it is also just how I interpret the code.
> Also the typo is fixed and it is reverse christmas again 😉
> 
> ---
>   drivers/net/ethernet/intel/igc/igc_main.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)

Tested-by: Naama Meir <naamax.meir@...ux.intel.com>