netdev - Re: [PATCH v7 bpf] xsk: fix immature cq descriptor production

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Message-ID: <CAL+tcoDn7HCW1+t0bceDPr2D-Q1EcSqh91eG3HJ+CjiLdX4Nag@mail.gmail.com>
Date: Wed, 3 Sep 2025 00:53:57 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Maciej Fijalkowski <maciej.fijalkowski@...el.com>, bpf <bpf@...r.kernel.org>, 
	Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	Andrii Nakryiko <andrii@...nel.org>, Network Development <netdev@...r.kernel.org>, 
	"Karlsson, Magnus" <magnus.karlsson@...el.com>, Stanislav Fomichev <stfomichev@...il.com>, 
	Eryk Kubanski <e.kubanski@...tner.samsung.com>
Subject: Re: [PATCH v7 bpf] xsk: fix immature cq descriptor production

On Wed, Sep 3, 2025 at 12:22 AM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Tue, Sep 2, 2025 at 6:39 AM Jason Xing <kerneljasonxing@...il.com> wrote:
> >
> > > > > >
> > > > > > > +               list_for_each_entry_safe(pos, tmp, &XSKCB(skb)->addrs_list, addr_node) {
> > > > > >
> > > > > > It seems no need to use xxx_safe() since the whole process (from
> > > > > > allocating skb to freeing skb) makes sure each skb can be processed
> > > > > > atomically?
> > > > >
> > > > > We're deleting nodes from linked list so we need the @tmp for further list
> > > > > traversal, I'm not following your statement about atomicity here?
> > > >
> > > > I mean this list is chained around each skb. It's not possible for one
> > > > skb to do the allocation operation and free operation at the same
> > > > time, right? That means it's not possible for one list to do the
> > > > delete operation and add operation at the same time. If so, the
> > > > xxx_safe() seems unneeded.
> > >
> > > _safe() variants are meant to allow you to delete nodes while traversing
> > > the list.
> > > You wouldn't be able to traverse the list when in body of the loop nodes
> > > are deleted as the ->next pointer is poisoned by list_del(). _safe()
> > > variant utilizes additional 'tmp' parameter to allow you doing this
> > > operation.
> >
> > Sure, this is exactly how _safe() works. My take is we don't need to
> > use _safe() to keep safety because it's not possible for one reader
> > traversing the entire addr list while another one is trying to delete
> > node. If it can happen, then _safe() does make sense.
>
> Jason,
> sounds like you're still confused what "_safe" suffix does.
> "_safe" doesn't help with concurrent access at all.

Hi, Alex.

Quoting Maciej to explain the function of _safe(): _safe() variants
are meant to allow you to delete nodes while traversing the list.

I meant the _safe is not needed at all as I explained above. The
af_xdp logic makes sure processes (like reading/adding/deleting) nodes
of this addr list are serialized. So why add _safe here, I wonder?
Just remove the _safe suffix then.

The moment you jump into the conversation, I feel I might get stuck
somehow, but I'm not aware of it... Please correct me if I'm wrong.

Sure, it's a trivial thing because it has no impact on the whole patch.

Thanks,
Jason