netdev - Re: [PATCH net-next V4 2/2] veth: apply qdisc backpressure on full ptr

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Message-ID: <87r01si4xr.fsf@toke.dk>
Date: Wed, 16 Apr 2025 15:58:24 +0200
From: Toke Høiland-Jørgensen <toke@...hat.com>
To: Jakub Kicinski <kuba@...nel.org>, Jesper Dangaard Brouer <hawk@...nel.org>
Cc: netdev@...r.kernel.org, bpf@...r.kernel.org, tom@...bertland.com, Eric
 Dumazet <eric.dumazet@...il.com>, "David S. Miller" <davem@...emloft.net>,
 Paolo Abeni <pabeni@...hat.com>, dsahern@...nel.org,
 makita.toshiaki@....ntt.co.jp, kernel-team@...udflare.com, phil@....cc
Subject: Re: [PATCH net-next V4 2/2] veth: apply qdisc backpressure on full
 ptr_ring to reduce TX drops

Jakub Kicinski <kuba@...nel.org> writes:

> On Tue, 15 Apr 2025 15:45:05 +0200 Jesper Dangaard Brouer wrote:
>> In production, we're seeing TX drops on veth devices when the ptr_ring
>> fills up. This can occur when NAPI mode is enabled, though it's
>> relatively rare. However, with threaded NAPI - which we use in
>> production - the drops become significantly more frequent.
>
> It splats:
>
> [ 5319.025772][ C1] dump_stack_lvl (lib/dump_stack.c:123) 
> [ 5319.025786][ C1] lockdep_rcu_suspicious (kernel/locking/lockdep.c:6866) 
> [ 5319.025797][ C1] veth_xdp_rcv (drivers/net/veth.c:907 (discriminator 9)) 
> [ 5319.025850][ C1] veth_poll (drivers/net/veth.c:977)

I believe the way to silence this one is to use:

rcu_dereference_check(priv->peer, rcu_read_lock_bh_held());

instead of just rcu_dereference()

-Toke