linux-kernel - RE: Re: Re: Re: Re: [PATCH bpf v2] xsk: Fix out of order segment free in __xsk_generic

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Message-ID: <20250610091125eucms1p3b7b0b19a533caffddce07c75596f3714@eucms1p3>
Date: Tue, 10 Jun 2025 11:11:25 +0200
From: Eryk Kubanski <e.kubanski@...tner.samsung.com>
To: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
CC: Stanislav Fomichev <stfomichev@...il.com>, "netdev@...r.kernel.org"
	<netdev@...r.kernel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "bjorn@...nel.org" <bjorn@...nel.org>,
	"magnus.karlsson@...el.com" <magnus.karlsson@...el.com>,
	"jonathan.lemon@...il.com" <jonathan.lemon@...il.com>
Subject: RE: Re: Re: Re: Re: [PATCH bpf v2] xsk: Fix out of order segment
 free in __xsk_generic_xmit()

> I've come with something as below. Idea is to embed addr at the end of
> linear part of skb/at the end of page frag.

Are you sure that this is safe for other components?

So instead of storing entire array at the skb_shared_info (skb->end),
we store It 8-bytes per PAGE fragment and 8-byte at skb->end.
Technically noone should edit skb past-the-end, it
looks good to me.

In xsk_cq_submit_locked() you use only xskq_prod_write_addr.
I think that this may cause synchronization issues on reader side.
You don't perform ATOMIC_RELEASE, so this producer incrementation
is atomic (u32) but it doesn't synchronize address write.

I think that you should accumulate local producer and
store it with ATOMIC_RELEASE after writing descriptors.
In current situation someone may see producer incrementation,
but address stored in this bank doesn't need to be synchronized yet.