Messages in this thread | | | Date | Fri, 30 Jun 2023 15:20:54 +0200 | From | Simon Horman <> | Subject | Re: [PATCH net v3] igc: Prevent garbled TX queue with XDP ZEROCOPY |
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On Fri, Jun 30, 2023 at 02:03:06PM +0200, 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 before locking, the outdated next_to_use is written 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@linutronix.de> > Reviewed-by: Kurt Kanzenbach <kurt@linutronix.de> > Tested-by: Kurt Kanzenbach <kurt@linutronix.de> > Acked-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
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