netdev - Re: [PATCH net] net: let skb_orphan

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Date:   Tue, 30 Mar 2021 16:39:23 +0200
From:   Eric Dumazet <edumazet@...gle.com>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net] net: let skb_orphan_partial wake-up waiters.

On Tue, Mar 30, 2021 at 4:25 PM Paolo Abeni <pabeni@...hat.com> wrote:
>
> Currently the mentioned helper can end-up freeing the socket wmem
> without waking-up any processes waiting for more write memory.
>
> If the partially orphaned skb is attached to an UDP (or raw) socket,
> the lack of wake-up can hang the user-space.
>
> Address the issue invoking the write_space callback after
> releasing the memory, if the old skb destructor requires that.
>
> Fixes: f6ba8d33cfbb ("netem: fix skb_orphan_partial()")
> Signed-off-by: Paolo Abeni <pabeni@...hat.com>
> ---
>  net/core/sock.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/net/core/sock.c b/net/core/sock.c
> index 0ed98f20448a2..7a38332d748e7 100644
> --- a/net/core/sock.c
> +++ b/net/core/sock.c
> @@ -2137,6 +2137,8 @@ void skb_orphan_partial(struct sk_buff *skb)
>
>                 if (refcount_inc_not_zero(&sk->sk_refcnt)) {
>                         WARN_ON(refcount_sub_and_test(skb->truesize, &sk->sk_wmem_alloc));
> +                       if (skb->destructor == sock_wfree)
> +                               sk->sk_write_space(sk);

Interesting.

Why TCP is not a problem here ?

I would rather replace WARN_ON(refcount_sub_and_test(skb->truesize,
&sk->sk_wmem_alloc)) by :
                        skb_orphan(skb);

This will get rid of this suspect WARN_ON() at the same time ?

>                         skb->destructor = sock_efree;
>                 }
>         } else {
> --
> 2.26.2
>