netdev - Re: [Intel-wired-lan] [PATCH net v2] igc: Prevent garbled TX queue with XDP ZEROCOPY

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Message-ID: <87a5wjqnjk.fsf@intel.com>
Date: Wed, 28 Jun 2023 14:34:07 -0700
From: Vinicius Costa Gomes <vinicius.gomes@...el.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,
 intel-wired-lan@...ts.osuosl.org, linux-kernel@...r.kernel.org
Subject: Re: [Intel-wired-lan] [PATCH net v2] igc: Prevent garbled TX queue
 with XDP ZEROCOPY

Florian Kauer <florian.kauer@...utronix.de> writes:

> 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 before locking, the outdated next_to_use is written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>
> ---

This patch doesn't directly apply because there's a small conflict with
commit 95b681485563 ("igc: Avoid transmit queue timeout for XDP"),
but really easy to solve.

Anyway, good catch:

Acked-by: Vinicius Costa Gomes <vinicius.gomes@...el.com>

Cheers,
-- 
Vinicius